Re: I don't want to undo what progress we've made, but. . .
I would go full time diaper-less. I was doing the same thing with my DS3 at 18 months and got super frustrated because sometimes he would go and sometimes he wouldn't, plus I was 7 months preggers- ugh! I restarted at about 23.5 months, one day after reading "oh crap. Potty training". Totally helpful ebook that go me over hurdles I kept hitting. He is now 24 months and is full time out of diapers, even at night. Today was his first day without an accident-YAY! We have to wake him at night to go potty: pick him up, pull down the pants (commando-style), say go pee, put him back to bed. He still has accidents, but it's not a big deal. Main thing is that he's learning that he needs to go pee. I think it's great that you instituted a prebedtime potty routine. I like what PP said about reading a book and then going one last potty time. Anyway, here's to more pottying successes for you and DS!