Going from 27.5 to 26 you could eke out a few extra tenths of an inch of tire width so if 2.6” is the limit with 27.5 if you size down to 26 you would probably fit 2.8 or 3.0 if you are lucky. The rest of your question gets confusing. Not sure what tire/ rim combos you are asking about when the bluto comes into play. One thing is i wouldn’t worry too much about geo changes, they may be noticeable but probably won’t ruin the bike. And you will never know what geo you like until you experiment.