The issues sound pretty small compared to the things you enjoy about this game/group. How do you know that once you quit and join a new game, that game won't be even worse? I say stick with it and make the most of it unless the game is simply boring.
Sometimes I wonder if people latch on to the idea of "railroading is bad" so strongly that they end up ruining their own fun. Railroading is not simply a bad thing. It is how the DM is railroading that is a problem.
A lot of players don't even consider that there is such a thing as "railroading the DM" which is equally just as bad. If the player is complaining because the DM doesn't let him ignore the adventure and run off to do his own thing, then I have no sympathy for the player. Just because the DM tries to stick to running the adventure does not mean the player can't still have fun if he doesn't deviate from that adventure.
Now if the DM pulls some shenanigans to thwart the players attempts at progressing the adventure simply because it was not mentioned that way in the written adventure, then I see a legitimate complaint. Even then, that still doesn't always mean there is a problem. There could be some valid reasons for thwarting those attempts that the player may not be aware of.