Is this position legal?

No, not legal. The pawn on b5 must have originated on A7, so how did the white bishop get to A6?

Looks illegal. White king must walk across rank 8 to get out via e6 (h6 is either occupied by the pawn or protected by the same pawn when it's retracted to g7). But this means removing bNa8 from the way, which means removing the wRb6 from the way. It cannot be removed along the file (blocked by pawns), neither to the left (wBa6 needs bPb5 to go, but this means retracting bPb6-b5, so the rook must go before the bishop), so it must go to the right, retracting bPd7xc6 in the process. But then e6 is protected, not to mention that bBc8 must be present (captured by one of White's pawns, not by the king)...

Yes its illegal ive tried out various variations.

Well, then how the heck will the rook get to b6?

1. Rc6-b6 d7xc6

Well, then try to find out the solution.

I agree.

Another way of explaining it:

With the bishop on b8, the king can only reach a7 from a8. But the king can only reach a8 after dxc6 is played, and before dxc6 is played the rook must be on b6, and before the rook rests on b6 the knight must be on a8. Therefore, it is not possible.

Is this possible?

Trivially.

Meant this one sorry mistake :/

Still trivially legal. Black's f-pawn captures something to move to e-file and promotes to a bishop.

So let me give it a little extra difficulty.

Legal? a) Position, b) +wPb3

EDIT : never mind, I just cannot count up to four. I left my reasoning to be the "show and gaze o' the time".

I see the intent, but not how it works out. Deleting the g2 bishop probably fixes it.

Illegal : the closest square to get a promoted bshop is c1 or e1, and the a or b pawn goes there by axb,bxc,cxd,dxe or c. 5 captures for 4 missing pieces -> nope, it's not even needed to look at promotion tricks for the a/b white pawns.

So I can't count to 12.

a) Position, b) +wPb3

My original intent is exactly that bishop.

Still illegal. White's abc pawns need collectively 4 captures (cxd, axbxc-c8=?, bxc-c8=?) to be fed. Black's missing usefully, NNQ (c8-B is useless.)

Okay I still can't count. I forgot that the pawns need to promote. My composing skills fall again.

No tricks and twins this time. This is the thing that I want to isolate, actually.

Trivially legal this time. fxexdxc/e, 3 captures. It's almost back to the state it was first posted in. :P (Also not mentioned yet is the bottom right which is trivial but needs just a moment of care.)

Better to think long and hard for composing. (And even when you do it can still fail horribly, like my thread bump.)

Nah, how the bishop, the knight, the king, and the rook play at the southeast is not that simple for beginners. (I should have made sure that both White rooks are original, otherwise it's too easy.)

...I think. Just to confirm: [Rook gets out from southeast, then knight goes to h1, then pawn g2-g3, then bishop goes to g2, then king and rook get back in is the intended solution.]

I believe that's illegal. 
Two of the white pawns are doubled, and black has only lost two pieces (both of their bishops). Thus, these pawns must take these bishops at some point in order to move into the next file.
The black f8 bishop cannot escape with the pawn formation (the f6 and h6 pawns block its exit), thus the white pawns cannot be doubled twice, hence the position is illegal. 

The bishop escaped before f7-f6.