Sasha,
Soberlink is a really good sobriety monitoring tool. The challenge might be getting your ex to use it.
Do you have an attorney? If so, you might want to talk to him/her about instituting sobriety monitoring when the ex has your child, given the documented and troubling incidents that have happened in the last year. Given your ex's record of trouble, a court might make him/her show s/he's a safe custodial parent. I think you would say ex no longer gets physical custody unless s/he demonstrates sobriety during those times.
You could try doing the same with a co-parenting counselor/mediator, if your ex is willing to meet with one with you. That would be less expensive and might get you better results. (My attorney said she didn't think I would have gotten what I did using the judicial system.) You would say the same thing: you know you're both loving parents who put your child's well-being above everything else; given your ex's trouble over the past year, you're concerned to know that s/he is safe and sober when the child is with him/her.
On the issue of your child wanting to spend time with the other parent--I'm in the same situation. My children are very protective of their dad, and they never complain or resist spending time with him. (Also, he has a bad temper, so they may fear ruffling his feathers, too.)