The DMG is way more clear that a monster casting spells is not "significant home court advantage" if we were fighting on the plane of fire, that would be one thing, but "you walk into a dungeon, since you are walking therefore automatically everything has to be lower CR" is not in the rules.
If you play any monster without treasure (aside from the ones that specifically say they get none), that isn't stock. The stock rules have, in the entry, a statement about treasure. A Pit Fiend without "Treasure: Standard coins; double goods; standard items" is not stock.
Yes, and the two ways to do that are:
1) You can't hide while attacking, because the sniping rules don't say you can, and the other rule is a general summary of penalties, not a specific rule statement.
or
2) You can hide while attacking, because that's apparently a rule in sentence 2, so you can just hide while making a full attack action and get off all your attacks.
Those are the two valid ways to interpret the rules so they are all validly applicable.
I too like to edit people's arguments out of their quotes when I'm lying about the rules to try to get custom 3.0 splatbook items into it because my character can't compete in core.