3 years and 3 months and regressing on potty training; co-parenting issue

My fiance and I have a very intelligent and willful 3 year and 3 month old. He began making great progress potty training a little before his third birthday and we were thrilled, and so was he. He was getting positive reinforcement from us, his nona, and his daycare provider, all of whom he would use the potty with, though not always happily. Shortly after this progress, however, he began to regress. He lives with his mother most of the time (my fiance and I have him at least one day and night per week and every other weekend), and she is resistant to presenting a united front or co-parenting with us on this issue. She also recently changed his daycare provider to a family member who is not too keen on putting in the work and time it takes to potty train.

I have two questions: first, though we plan to approach his mother once more about working on a mutual strategy, if one can not be decided upon should we back off the potty training entirely? He is still wearing regular diapers with his mom and is even laid down to be changed when he pees or poops his diaper (while we have him in pull-ups and he changes them by himself with only a little help). Is it better to wait for her to take action, or could the small amount of time we are with him really make a difference?

Second: The one incentive he has really expressed interest in is big boy underwear; he picked out his own at the store and immediately wanted to abandon his pull-ups for them. I have been a little resistant to this because of the mess an accident would cause, and I was hoping for some use of the potty before we switched to regular underwear, but could the responsibility be just the incentive he needs?

I am looking forward to responses as they will help me with two crucial issues: co-parenting and potty training!