Will 2 2030 CE stack?

Yes thats how it works and is acrualy rather effective

Yes, it is. But not 18 stars, 16 stars for non-MLBed and 20 stars for both MLBed 2030.
Each single 2030 Can give you 8 Crit Stars, if MLBed it will give you 10 Crit stars.
3x 2030 CEs and it will give you 24 stars, 30 for 3x MLBed version.

A CEs that don't really needed to be MLBed to function properly, even the regular 2030 is totally great.
I think, what do you mean is a non-MLB 2030 + MLBed 2030 which gives 18 Crit stars.

My favorite is with a single Hans and a 2030, it will give me 17 stars.