I'm not sure that I'm qualified to give you full advice on this, but I'll go ahead anyway 😆 .
1) The difference in volume between a 300mm length of 25mm by 25mm baton and 20mm by 20mm is 0.0675 litres, so if you multiply this by the length of the baton you're using it'll tell you the difference in volume. Personally I can't see it making a massive difference, but I expect somebody will be along soon who has better knowledge. What's the total length of baton you'll be using? (I can't remember the size of the cab).
2) I'd glue and screw.
3) yes, that's easy and feasible. You just need to wire the plates so that plugging in the second speaker connects it in parallel to the first by shorting between the positive of the first post to the positive of the second and the same for the negatives. You'll need to make sure that you never try and connect two amps to the same cab, or to the two connected cabs. I've been thinking about doing the same if I ever get around to building this cab, but would also quite like it to be configurable to run in stereo for my keyboard. Hmmm,maybe I need a four box solution 😆, but then again I already have an Ashdown 4x8.