2. I'm afraid that the "camouflage" ability seems to be very heavily bugged. I've tested the follwing aspects in hotseat mode (playing both sides) - otherwise you can't tell what the enemy sees. Here are my findings:

• Camouflaged stacks are visible to the enemy. The only thing that changes is that the enemy does not get to see the picture of the top unit. But he can still hover over the stack to see exactly which units are in the stack (see also the following post on this issue - it's a more general issue, not restricted to the camouflage ability).
• When a stack enters a region in which there is a camouflaged enemy stack, a discovery test is triggered. That's good. However: if you don't move directly into the region, but only "through" the region, no discovery test is triggered.
• If you have a stack in the same region with an opposing camouflaged stack, there is no automatic new discovery check at the start of the turn. But - at the start of the turn/when the stack has not moved yet; I've not tested whether you can also do it after some movement -  you can initiate a new discovery check by moving your stack "within" the region. Click it and place it at some other spot in the same region. 
• You can try to trigger several discovery checks by moving a unit in and out of a region.
• When a discovery test succeeds, no battle is initiated (regardless of whether the "attacking" stack has a leader or not). Both stacks stay in the region and nothing happens.
• When there are opposing stacks in the same region, no stack can move on and no battle occurs. The stack with the camo-ability can re-activate the ability  - only then both sides are free to move on. 
• Unsuccessfull discovery tests can lead to weird situations. It can happen that there are two stacks of one side (the first entered the region and did not succeed the discovery test, the second stack succeeded) and one (formerly camouflaged)  stack of the other in the very same region and no battle occurs. 
• In the game, all units in a region are merged together into one stack at the end of a turn. This is very unconvenient and also leads to weird situations where camouflaged stacks are merged with non-camouflaged stacks. 

Other aspects of the ability seem to work as intended?

• You can't un-camouflage a stack in a region where enemy stacks are present unless the camouflaged stack has a leader - HOWEVER: I have not tested whether this really initiates a battle. 
• To enter a region in which there is a camouflaged enemy stack, your stacks don't need to have a leader (even if you succeed the discovery test, no battle will occur, regardless of whether your stack has a leader or not).

3. Fog of War in hotseat games

Both in hotseat games (you vs. you) as well as in regular games (you vs. AI), the very first turn of the faction that goes first in the scenario works fine: You can hover your mouse over enemy stacks but no info-tooltip pops up. 

From the first turn of the other faction (that goes second) on, things change: In regular games, everything works fine (note though that now you will get a popup that tells you the faction's name if you hover over an enemy stack). In hotseat games, however, you can hover over enemy stacks and get all information - you just need to hover over the stack a bit longer, and an extended tooltip will pop up, telling you the names of all the units in the enemy stack. 

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Another big, somewhat related issue I want to point out is that you regularly are informed about the composition of your enemy's stacks when they're unsupplied or laying siege. Even if it's the AI's turn, the "unsupplied units"-window pops up, telling you which stack (with the exact list of units) are unsupplied in which region.

4. Stacks solely made up of supply wagons consume supply like ordinary stacks: When they're in an unsupplied region, they loose one of their supply units. Setting the supply units to "is not supply dependent" doesn't help. As soon as you tick "is supply", "is not supply dependent" doesn't work anymore.  Is this intended?

I can't get custom music to work. I've placed it in the music folder (in .ogg format) and set the editor (edit scenarios/sounds) to "replaces default music", but it doesn't play in the game.  

I'm glad to hear that! Music makes the atmosphere! :)

Some more issues rather than bugs:

• Very tricky and a bit confusing when you first encounter it: Cards that apply an effect "until the end of the turn" apply the effect until the end of the game-turn (the turn as it is indicated by the turn-number in the top right corner of the screen). It sounds logical, but it's not really intuitive. You would expect the effect to last for this player's turn and then the next player's turn, in other words: a turn-sequence that consists of 1 turn for each player. Right now it works based on the game's turn, which might very well end with only one player finishing his turn. So right now, players might get advantages or disadvantages depending on whether they go first or second in a scenario. There are ways to work around that problem, (setting up a card for the other side which applies the effect until the "next turn of the specified side") but it is quite tedious. 
• A request: A button to duplicate a card effect. Even better: an option to activate a card (check the conditions, apply effect if conditions are met) every turn. Right now, you can only apply the effect permanently (so the conditions are checked only once, when the card is first played, not every turn).
• It's really fiddly to play/activate cards. Somehow the window to aim your mouse cursor to play the card is strange and/or too small. (I will create a small video with some of my tests in which I can also point that out).