Jennifer,
If the two different kinds of mix are reasonably similar in ingredients and coarseness, I wouldn't be too concerned about them not wicking water back and forth. That probably won't be a problem.
I certainly do over-think everything - that's my style and one of the main joys in my life.
I don't know wintersowing, but aren't there drain holes, and something to keep excess rain off? If the mixes drain reasonably well, shouldn't that drain well enough to let air back into the mix?
If you're still worried, maybe cover them during heavy rains to keep them drier.
Next year, if the mix you buy doesn't look like it drains well, you might add Perlite or grit or anything coarse to help its drainage.
By all means, for sure and certainly, please go by your experience and not by the perhaps wacky way I work around my own eccentricities.
Boil down everything I said into "seedling mixes should drain well enough to let excess water out and air in".
I think the more experienced people just avoid over-watering!
(Now this IS over-thinking, but if you're still worried, sit the jugs on top of something water-absorbent like a cotton sheet. That will wick excess water out of the jugs and help it drain away, even IF the mix is so fine that it tne4ds to get water-logged.)
