If you're seeing Plan C as "letting him do or have what he wants" then you've not quite got the right concept. (I hope this isn't how Ross Greene words it himself - I've yet to wade through the new edition!).
As I see it - you make a list of the behaviours you really want changed. You then sort them into various categories as follows:
Plan A (what I learnt as basket A, from the earlier edition) - only stuff connected to immediate safety (such as grabbing a kid as they're about to run in front of a car), and school attendance. You will do ANYTHING to get what YOU want here, even provoke a meltdown if you have to.
Plan B - about three or four behaviours only, that you think you have a chance of working on. You want to work with difficult child on improving these behaviours, but you will back away from provoking a meltdown rather than insist.
Plan C - everything else. Because we can't do it all at once. We choose our battles, basically. Plan B is the specific battles we've chosen, Plan C are the battles to come when we've accomplished Plan B's first list.
As you achieve improvement in the Plan B behaviours you move them on and adopt the next ones in line from Plan C.
How you use it:
If you have your own specific list (have it written down somewhere accessible to you, don't stick it up for all to read, including difficult child - counterproductive) then it's easier to stay in touch with what you want to work on.
You use whatever means (preferably not punishments - they tend to not work anyway) to get what YOU want in the Plan B areas. The Plan C stuff shouldn't even get mentioned. If it does, you have just made it Plan B. (So if you feel he's having what he wants, if it hasn't been discussed and therefore put in Plan B, it shouldn't matter).
If you are trying to negotiate on improving a Plan B behaviour and you are losing, the recommendation is that you walk away. You back off BEFORE the meltdown happens. To most of us this feels like we're giving in and letting difficult child get away with continuing bad behaviour, but you have just sent a strong message to difficult child - your main aim right now is to help him avoid meltdowns, but your secondary aim is to improve those behaviours.
Our difficult children do not like raging. They hate how it feels. If the raging continues for years, some of them get a secondary gain from the sense of power they feel that raging gives them; the fear they can induce in others. That's why it's really important to stop this before it gets that bad and they're old enough to get a power feedback from raging.
So difficult child is going to soon recognise - you are trying to help him stay calm. And if you're prepared to risk that to improve a particular behaviour, it really is important to you and may be worth his efforts. It's not altruistic on his part - when he recognises that you are trying to help in the one area where he KNOWS he has a problem, then there is deep gratitude, believe me. They may not be aware of it; they may at first misinterpret it as you being extremely indulgent, but the message is going to be getting through, even at some subliminal level.
You shouldn't have too much in Plan B, and remember - when anyone tries to correct a behaviour, it has automatically been put into Plan B. When anyone forces that correction to the point of meltdown and beyond, it's been put into Plan A. Changing the plans around slows down the rate at which difficult child 'picks up' that you are trying to avoid meltdowns, so it slows the whole process.
An example: difficult child 3 insisting on eating with his hands and then wiping dirty hands on his clothes. This was Plan B for us. We also tried to work out why and we know difficult child 3 has problems with his joints as well as poor coordination.
Before I had muscle weakness problems I happily used chopsticks but now my hands get tired easily. For difficult child 3, using proper utensils is partly a fatigue thing, partly laziness. So we fed him meals where he could use a Splayd (aka spork, I believe - an Aussie invention from over half a century ago). I also provided a paper napkin and whenever he was reaching for his clothes with his hands, I shoved the napkin in there instead. But this has been a very difficult behaviour to change - it is STILL in Plan B.
Toilet training (for bowels) - that would have had to be Plan C because he simply didn't have the ability to do anything about it. When we finally chose to work on it, it would have had to be the only thing in Plan B. We used a reward system (a box of M & M's glued to the toilet wall were to be his when he finally went one day with using the toilet for BM and cleaned himself properly. Then it was one box for a week of being clean and using the toilet).
Homework - ALWAYS Plan C. If the child asks for help it becomes Plan B but it NEVER becomes Plan A, which seems to be where most schools want it to be.
For us, with difficult child 3 doing a version of home schooling, homework is actually schoolwork and it's Plan B. But we cope with it by having a strict rule in place - "school work for school hours". Any other lack, although I will help him, support him, remind him, I will not nag him because the ONLY consequence for failing to do his work is that HE has to explain it to the teachers. And he HATES that. I praise him a lot for getting his work done, but it's not over-the-top praise, it's "Doesn't it feel great to know you have that work under your belt now, you can move on to the next topic and feel very satisfied with yourself."
Tidying their room - Plan C. It's a ghastly mess and you can't even shut the door, but it's THEIR mess. I refuse to enter the boys' room. I refuse to enter the girls room, although you can still see floor in there. But if any of them begin to tidy, or I suggest a bribe, I happily wash and sort any clothes they unearth (usually thoroughly outgrown by now).
You may have different ideas about what you want to work on - it's your choice. But if you find that a certain behaviour just doesn't seem to be improving, then put it back into Plan C because it's likely the child simply hasn't got he ability yet to make the changes you want. A good example here is if you want to put swearing, bad language or aggressive behaviours in Plan B - these behaviours are too closely connected with the raging itself and the reason they rage is because they CAN'T do anything else. Don't set yourself up for failure.
For us, the language, the aggressive behaviour, the raging doesn't even come under Plan C - rather, it is what ALL the plans are working on, but obliquely.
I found the longest time was the first - plus, getting everyone else in the same household on board. Those who are NOT on board become the focus of difficult child extreme resentment. They don't know why, but they often say they hate that person. With a forceful adult refusing to use this method while someone else IS, that forceful adult is going to be puzzled, angry and very frustrated. You then have to be careful to not fall into "good cop, bad cop" patterns.
Do what I did - summarise the book and the method for your husband. Mine wouldn't read the book either - not that he was being difficult, it was something he just couldn't get into. My other adult kids also had to come on board and no way would they read the book - getting them to read ANY non-fiction is a huge ask. The summary I did was great for explaining to them. Plus, I could make them shut up and watch.
My husband has always been a strict disciplinarian. This not only wasn't working, it was causing problems. When I started to read "The Explosive Child" I hadn't even completed the book before I was using the method. husband thought I was crazy - how could it work? But I began to see improvement in difficult child 3 very quickly, although it took a while before I could 'graduate' any behaviours from Plan B and put in some new ones from Plan C to work on.
However, husband quickly became an extreme focus for difficult child 3's hostility, which I think slowed down husband's acceptance of this method. We've had to really work at building their relationship back with 'boy time'. husband is now calmer, he feels more in control of the whole situation (which I think was a huge issue for him) and he and difficult child 3 now get on well. They still have their differences - they're both fairly reactive and difficult child 3 will be so for a very long time, some things we can't change quickly. husband has had to bite his tongue a lot, and hates doing it although he can now see the reasons for it. it IS stressful for him and I do worry about him being stressed. But I know a return to frequent rages makes husband even more stressed.
Once you get into the habit of this it becomes a knack. You WILL make mistakes and they WILL set back progress, but the more progress you make the less setback you will have for each mistake. Don't beat yourself up over it, just pick up where you left off, be prepared to apologise to your child if you make a mistake (it's not crazy, it's showing respect which you need to do in order to teach respect) and make sure you each have an escape hatch for when you need to walk away a bit further than usual.