1) Go download an auto-clicker tool, it speeds up your gain of any raw materials. My auto-clicker does 2 clicks per millisecond. I usually travel back to previously cleared floor and it takes only average 2.8s to clear a floor.
2) Save your gems for miners first, they are very useful to be unlocked early. After that, you should be upgrading the forge output and finally other gem upgrades. (You will unlock every gem content eventually)
3) Upgrade forge first, it helps the most grinding part of the game. (Legendary gems cant be transmuted using alchemy and it is needed for a lot of miners' last upgrade)
4) Alchemy is your best friend. The essences can be gained easily using auto-clicker in Scrapyard and Dragon Mountains.
5) Prestige whenever you like, it helps you to advance to later stage easier.
6) Miner setup is casual. Choose whatever miner that helps your progress (double essences/raw materials/instant smelt etc). Anyway, if you use an auto-clicker, it doesn't matter much, because your clicks clear faster than the miner walks. Anyway, I suggest to unlock Leon as soon as possible, he is useful in any setup.
7) Miners' passive won't work in offline mode.
8) Offline gain depends on your deepest floor and time. Travel to desired floor before going offline won't help.
9) The abyss is for late game players. Don't spend any resource there unless you have no other use for that particular resource. Believe me, it is a dark hole there, so far I can't see any ending in the Abyss.