Two questions about how the en passant capture works

Question

I am still confused about the en passant rule. There are two possibilities regarding the capture itself that are not clear to me.

[FEN ""]
1. d4 d6 2. d5 c5 3. dxc6 b5 (3. c4 e5 4. dxc6)

Scenario 1: In main line shown above, is it possible to capture via en passant again after 3... b5 with the move 4. cxb7ep? In other words, is it possible to capture en passant twice with the same pawn?

Scenario 2: As shown in the given side variation, is it actually possible to capture the black’s pawn on a later move? Note that although the black pawn is still in the diagram, you must imagine that it isn’t.

Answer 1

En passant can only be played when the enemy pawn moves over your pawn's capture square during it's first move (moving 2 squares); so the answer to both condition 1 and 2 is no.

It is not possible for a pawn to en passant twice in a row.

My understanding is in early chess pawns could only move one space at a time, so when the 2 move rules was added they also added en passant to counterbalance the fact that you might lose a chance at capturing your opponent's pawn since he could now move over your capture square.

So, in essence, because after capturing en passant it is no longer possible for an enemy pawn to pass over your capture square, it is no longer possible for your pawn to capture en passant.

The official FIDE rules, article 1 section 3.7d states:

A pawn attacking a square crossed by an opponent’s pawn which has advanced two squares in one move from its original square may capture this opponent’s pawn as though the latter had been moved only one square. This capture is only legal on the move following this advance and is called an ‘en passant’ capture.

Answer 2

A pawn capturing en passant (the d pawn in this case), basically interrupts the passage of the adjacent (c) pawn on his third rank (c6 for you), on the way from c7 to c5. This privilege can be exercised (retroactively) only on the immediately following move.

Once the c pawn has arrived at c5 (without interruption), there is no "second change" for the d pawn to capture him.

Answer 3

If, in the original diagram, above, Black's move had been 3. ... b6, White could not have captured it with the pawn on c6. Why would you think that because Black played b5, the c6 pawn could do so? It is simply physically impossible for a given pawn to capture en passant a second time.

Answer 4

This is just a bit of though experiment of mine regarding the rules of chess. Enjoy!

While a pawn can only capture en passant once in today's rules, as @Seth points out in his wonderful answer, it wasn't always so cut and dry in the 19th century. Many joke compositions have been made that exploited the fact that you could apparently promote to an enemy piece. However, promotion into an enemy pawn is a bit unclear, because there is no rule on how a pawn on a player's first rank would move.

But I think that since a player use to be able to not promote a pawn and keep it frozen on the 8th rank, it makes sense to me that a pawn on the first rank should simply be able to move forward. Since it would be it's first move, it would make sense to also grant it a double step. Lichess's supported chess variant Horde Chess allows it for the the first rank white pawns, as an example.

As such, if you could consider a pawn that promotes into an enemy pawn as the same pawn after the promotion, then a pawn could capture en passant twice. It's also a slight matter of chess semantics as well when it comest on considering it the same pawn.

Firstly, the black pawn captures a white pawn en passant and goes to the 7th rank.

[FEN "4k3/6p1/8/8/6p1/8/7P/4K3 w - - 0 1"]

1. h4 gxh3 2. Ke2 h2 3. Ke1 

Since an illegal promotion to an enemy pawn can't be shown, let's presume that it happened. The black pawn moves onto the 8th rank and promotes into a white pawn. The new white pawn advances and captures a black pawn en passant. (I can't show it having a double step from the first rank, unfortunately.)

[FEN "4k3/6p1/8/8/8/8/8/4K2P w - - 0 1"]

1. h2 Ke7 2. h3 Ke8 3. h4 Ke7 4. h5 g5 5. hxg6

Now you have seen a pawn capture en passant twice, given that you bend the rules a little, an if the promoting pawn is considered the same pawn afterward.