what happens if lillia fae fawn is defending. and moves with smoke and mirrors away with a sprite. a new sprite appears. and a sprite is moved in. but at base they have mask of foresight. does one sprite get +1?

Yes — the Sprite that swaps in to replace Lillia gets the +1 from Mask of Foresight.

The Mask's "attacks or defends alone" condition is re-checked when the Sprite enters the battlefield during the showdown as the new defender. At that moment it's alone (Smoke and Mirrors has already resolved, and Lillia's token trigger is still waiting on the chain), so Mask triggers anew for it and locks in. The order you resolve Mask's trigger vs. Lillia's "play a Sprite token there" trigger doesn't matter — even if the new 3/3 token appears first, the +1 was already earned and isn't removed. (The new token itself doesn't qualify, since it doesn't enter alone.) (FAQ 11212 / 10888)