I wouldn't lose too much sleep about the accs layout provided you are willing to find other rt hand chord notes in opposite direction, and to explore alternative fingerings for note runs that include accs.
Long time since i looked at book 1, but recall it being mainly "on row" stuff. Basically they get more and more complex and look to have you playing in several off main row keys by the end.
They carry C/Fs with a full accidentals row on stage, yet a lot of their repertoire is in various modes of D, be that major, minor or minor with an Eb (phrygean mode)
A full keyboard like that can technically reach any key, but some just lie there, some are frankly "pigs".
Get to know the modes of your main rows really well - G, C, Am - and branch out from there. The G# is there to make A harmonic minor work, common in French folk, so you should probably have the G#/A# plate their way … easy enough to turn over inside, or "any local melodeon fettler" will do for a small fee
Yes, their stuff is different. They started out very trad, Central French and Morvan stuff, including "bells and ribbons", but Stéph in particular had been a prolific writer of tunes. Both have jazz tendencies, explored in the later books