Okay I’ll try to keep this as short as possible ;-)
1. Not at all. I personally don’t think map changes are useful because most players unbalanced things more than they balance them. If you like bigger games, I can suggest trying the Anniversary Edition, Europe 1940 or Pacific 1940, or Global 1940 (Europe+Pacific together), but for the start Revised is very good to get used to the game.
2. A bid gives a side, either Axis or Allies, additional IPCs before game start, which they can use to buy units that are placed on the board before game start. This is done to balance the game. Revised mostly uses a bid of 8 or 9 for the Axis, whereas e.g. Anniversary uses an Allies bid. The units purchased with the bid money can be placed in any friendly territory usually, but only 1 unit per territory (that’s common but upon your agreement). When you bid for a side, say Axis in Revised, you could e.g. bid 12 for Axis, your friend goes lower to 11, you say 9, he says 8, then you say you pass, so he gets Axis and a bid of 8. So the bids go lower and lower until the sides are determined.
3. No idea ^^ I am not really good at that.
4. Just AA guns may fire on the bombers. Fighter interception is not possible in bombing raids. This is a reason why you may want to use the LHTR heavy bomber rules (then heavy bombers choose their best dice of the 2 they have in attacks, the better one +1 for bombing raids, but can’t take both) if you play with tech because heavy bombers are a bit overpowered.
5. Yes, kinda. If no destroyers are present, the subs have their first strike ability. So the subs of both sides would fire first in your first turn of combat, then casualties are taken (note: must be sea units), then the remaining units fire. After the first turn of combat, surviving subs can submerge and then not take part in combat anymore (also can’t be taken as losses then anymore). If they don’t, repeat the same process as above, subs first strike, then other units fire, for the remaining combat. If a destroyer is present, it prevents enemy subs from submerging and takes away their first strike (so you can fire back with all units, even when the sub hits some they still get to fire back) as long as the destroyer is alive.
I hope this answers most of your questions
Darkman