My first piece of advice would be to talk to an Texas lawyer for credible advice.
Second: Most of your questions about what the judge can do are at this link (IMO):
[link to law.onecle.com]

It says the interlock can be placed on a vehicle owned by the defendant or the vehicle most regularly driven by the vehicle. So you may have to live with the timer if that is the vehicle he regularly drives.
It might, if possible, be an idea to get a second vehicle registered in your name that he does not regularly drive. I don't see where the court would be able to force any sanction on you. (Unless of course you let him drive it and gets caught.) But that's just an opinion and I refer you to my first piece of advice. I have know idea of any particular case law that might contradict the law linked.



Is that you, D.? Thanks. I will head for the link. If the device had some way for it to determine who is driving, could lock him out, and let me drive, that would be one thing. But to lock the innocent away from using what is also my property can't be legal.
IMO, it's reckless endangerment, a violation of my civil liberties, and violates joint property use law at the very least.