It was a fun experience and I think your kit is a great product, so I thought I’d share the story.
The schnozzle bag works ok with an adapter. At the moment the adapter is a tiny bit loose on the bag and a bit too hard to remove from the red press-fit valve. Small adjustments to the dimensions would fix this. Making it from white POM was probably not the best option in the snow, I will add a piece of string to it. It requires a bit of force on the bag to get the air past the “flap” in the valve, but I think it is ok. Haven’t tried emptying fully through the valve since I have the zipper, but guess it would be a bit slow. Putting something through the valve to keep it open when folding/rolling the raft could be needed. In general I don’t think a few minutes more or less for inflating or deflating matters much.
I haven’t decided what to do yet, but a top-up valve in the seat sounds like a good idea. Haven’t added it to the raft yet. One option I’m considering is to cut open the plug from a press-fit valve and glue it to an 80 l dry bag I have for my backpack. It would need a plug to keep the bag waterproof, but it could be an efficient filling bag for the raft. I could then stuff the backpack in the dry bag, fill it with some air and use it for a seat. Not sure if the bag is dog-proof, but it would give several uses for one bag. Having the schnozzle bag for backup is good anyway.