Rob,
You are not overreacting. You have a limited time to make a decision about what you will wear and are going about it in a methodical manner.
The sizing discrepancy could account for the leak, as a snug fit at the crotch is the most important factor. However, an increase in hydration could account for it.
Whichever diaper you choose to wear, it is important that you feel confident and secure with it. The other diapers you mention both are good diapers.
Also, recall that I add a small booster pad to my diapers and reinforce the tapes so that I can tape them snugly. The small NorthShore Care booster pad holds 15 oz.! That adds more than an hour between diaper changes.
I wore the Seni plus a booster pad combination last night (under my cloth diaper) for over 10 hours without a leak. I was relatively well hydrated, but not over hydrated. However, there are too many variables to be able to compare our experiences.
Re heat and layers, I would skip the boxers and keep the diaper, plastic pants, and onesie. It helps disperse body heat if your "plastic" pants actually are breathable PUL pants.
What do you think?
--John