Re: Will part-time underwear be confusing?
We did part-time EC for ours too. He self-trained for poo at about age 6 weeks (had maybe 12 poopy diapers from 6 weeks to 16 months or so). I had him in cloth diapers during the day for pee until 22 months. Then, we just switched to underwear. Now he is 24 months. He STILL has a lot of accidents, especially in the mornings (not sure why, he seems to hold it better in the afternoon). If we forget or if he gets off his pee schedule, then we have surprises.
So, even if he "gets" that he's in underwear, he still may not tell you when it's time to go, especially as he gets older (strangely, my LO told us when he was younger, but has since stopped). Why bother with underwear anyway until you are ready to take the plunge to full time underwear? You can still rush to the bathroom, offer frequently for pee, etc., with trainers, right? And, if he really likes the underwear, he might start refusing the trainer, which might require you to take the plunge to FT underwear before you are ready.
On the other hand, I definitely don't think it would hurt! Just do what is easiest for you, he will eventually get it.