1. A mana generator is indeed a single L/O/anything gem. You need to start on a level with L or O to have an easier start with this. But if you would start with O only, or you would have a wild gem (I don't have access to the skill yet), it gives some mana and could even appear as L/O, so just use this ;)
2. Do challenges and collect journey amulets. If you are not premium, it's quite possible that you won't be able to unlock corners.
3. Yes, I think, at least I have troubles in opening fields - I have once resorted to having a level run with swarm only, otherwise I couldn't win the level - even at 1.00 difficulty. However, the farther into the game, the easier it could become, especially if you will learn how to build mana generators with a good initial setup.
4. Haven't found them yet.