Hello there! Thanks for playing, enjoying the game and providing some feedback!
3D2D is meant to be replayed, and with repeated play you'll not only learn the levels and enemy positioning but will start to take advantage of the environment and the various paths as you play with different characters. In terms of difficulty it is closer to Sonic 2 SMS which was harder than most, but very rewarding once mastered.
The mid air mechanic is exactly like in the mega drive classics: you don't have air control if you jump after a spin dash. Don't forget that Sonic has the insta shield as his ability, which restores air control when used. He is the only character that can regain air control after a spin dash in sonic 3 & Knuckles.
The save game system is a single slot, so you lose your progress on a new game. However it remembers how many extras you unlocked, and once you unlock everything your save game will become an "All clear" save, and you'll be able to choose any character and any level without restrictions.