Barrel's visit at your house is indeed repeatable, already had it couple of times.. I hope one day more characters will visit.. (like Gunnar or Bernard :p)
I haven't started with the Nameless arc yet... can he... really rewrite your savefile??
if you fail - you cannot revert the savefile - so you cannot return - and i think it will corrupt all your alternative savefiles too - so you cannot enjoy some cthulhu fun and the go good route - but there is alternative - someone wrote about it here - something about making copy of your save or something