Playing with Austrians and reserve Cuirassiers, I finally found a really good solution for being cut to bits while I waited for my monotonously useless sub general to arrive. I switched to using 4 Valiant Hussars instead. OK, that advice does not help a British player...
If you are playing 50mm bases on 8x5 tables then the 28mm game has slighly more (relative) room than playing 40mm bases on a 6x4 with 15mm. So the number of support choices should not be an issue between 15mm and 28mm. If you are not using 50mm bases on an 8x5 ... then you should!
My own view is that reserve options in the AB lists are generally no more powerful than an organic option - it is just that some troops are only available as a reserve. I would certainly not recommend cutting back the number of reserve units you get to bring them on early - they are not that strong to start with.
No idea how to fix the Pen British list, but boosting the Dutch-Belgian cavalry ally to a reasonable force would fix the Hundred Days list. Pen British may just be an oddity - you have to take more infantry support, hope to attack, and then hole up somewhere and hope the other guy will impail himself.
Cam