Personally, I think house arrest is a set up, especially for teens. Few succeed, even when very motivated. It is just too hard. I've advocated for youthful offenders for many years. Time and again I've seen a teen try and fail. If a child thinks that juvie is preferable to being home, then there is no way it will succeed.
Here's hoping that his PO doesn't give him what he wants and sends him back to juvie. For kids like yours, I've recommended things like Residential Treatment Center (RTC);,, juvie and work program (boy, did they hate that idea!); back to home arrest with a work program and a personal PO who would check on them daily. I'd been know to pick the kid up every day at school and take them to the local soup kitchen to work. The thing worked the best and actually sometimes turned the kid around was an Residential Treatment Center (RTC). Another possibility is a youth camp. These, however, just seem to teach kids more crimes and fuel the anger.
As to your mother in law, she is a voice not worth listening to -- just remember where her parenting has taken her son. Alcohol is frequently an escape. Sometimes from the inner mind, sometimes from the outside world. It sounds like your husband had a lot to escape from being raised by that woman. Don't let her drive you in any way, shape or form. She's truly not worth it.
As for your phone, sounds like it is time to password it so that he can have no access.
(((((HUGS)))))