When ALL 3 lanes are losing, it's hard for me to choose which lanes to gank more importantly.
This is a really difficult and highly contextual question to be asking. Generally speaking, save the guy who has the best chance of carrying the game (Hyper-Carries, huge waveclear that can stall the game, etc.) Also analyze why everyone's losing. If everyone is just plainly less skilled there's not that much you can do to be completely honest, but if it's a matter of the enemy Jungler ganking a lot for example, suddenly Counter Jungling or Counter Ganking options open up if you can track the enemy Jungler.
The excessive amount of "Please gank my lane" from all 3 laners at the same time.
Just mute everybody. At that point people aren't going to be nice or helpful to you.
The moment when you waste your flash to gank a lane and the enemy flashes so they escaped, should I come back to gank or what?
Yes
When you ganked top but need to farm to catch up leveling but your mid laner/Bot laner are begging for help quickly so you're behind in CS/level against the enemy jungler.
So this is a bigger issue in Solo Q but generally speaking, especially if you play Junglers with high Utility or are known for early game snowballing, CS/Level leads in the Jungle isn't actually as important as it is for laners. Your job as the Jungler is to control the pace of the early game. It's not your job, typically, to hypercarry Mid to Late game. Between getting Raptors or a good opportunity to snowball your laners, snowball your laners. This does not mean drop everything and bend over for your laners, especially if your laners aren't that good to gank or skilled players as a whole. What this means is that if you see an opportunity, take the initiative to gank lanes instead of just farming for the sake of farming.
- Enemy champions with insane mobility that makes it hard to gank, is it worth ganking them?
Yeah, if you know how to. Again contextual.
You seem new to Jungling. Get a lot more repetition in and then reevaluate yourself after every game. Some of these answers are just a matter of experience and context.