The AC drain pan should already have a P-trap associated with it. That's because there's a pressure difference between the inside of the AC unit and the conditioned space. Just tee in downstream of that trap and you'll be fine using that pipe as a path for your condensate that avoids a new wall penetration. I don't see why your dehumidifier itself would need its own trap.
However, do not tap into the upstream side of the AC P-trap, and don't drip directly into the AC pan.
It's all up and running. I did have to use a pump to get up about 18" and across the crawl, but I tee-d into the condensate drain about a foot inside the crawl and used a threaded barb fitting.
I also noticed that the condensate drain for my 1st floor a/c has a big belly in it, but I don't see a great way to fix it without totally rerouting it. I may just change that to drain into this condensate pump as well and call it good.
The aprilaire dehumidifier I installed has a pump cut out, so that won't run if the pump fails, but I don't know if I can do anything about the a/c