Eight men on the line
 

With eight men on the line of scrimmage, the tight end (A-23) lines up next to the tackle, but is covered up by a wide receiver. During the play, A-23 remains at the line and blocks a blitzer. Is this play legal?

Legal.

The rule states that you must have at least 7 players on the line of scrimmage (FED) or no more than 4 in the backs (NCAA).

There's nothing that says that you can't have 8, 9, or 10 players on the LOS. (You can't have 11 on the LOS because if you did nobody could receive the snap).

Now, if A23 goes more than 3 yards downfield before a legal forward pass is thrown, then you've got a problem.

If A23 lines-up between "the tackle" and a "wide receiver (who is up on the line) he's NOT A TIGHT END, he's simply another LINEMAN, between the tackle and the player on the end of the line.

There is nothing illegal about this formation in either NFHS or NCAA. As was mentioned there is a risk of an ineligible lineman downfield penalty if he travels too far and a forward pass is attempted, but that's true of any lineman.

This formation gets flagged on Sundays sometimes, I think because they have rules that certain numbers must be eligible unless they announce them as ineligible.

I've seen that done twice in a game by a team that was getting walloped & conceding safeties by snapping thru the end zone.

Happens all the time in an overload formation. Coaches will sometimes even mention it in pre-game.

I think that's the part I'm unsure about. Should it be flagged?

So the reason it gets flagged is that the NFL requires that all of the players between the players on each end of the line be ineligible whether by number or after having reported as ineligible. So when this gets flagged it is usually because a player with an eligible number is “covered” by another player on the end of the line. The penalty is still considered an illegal formation, but it isn’t specifically for having 8 players on the line.

NFL rule set can be totally different in some instances.

If either the TE or WR shift to the backfield before the snap, is that legal, and does it make the TE eligible to go downfield?

Under the NFHS Rule Code, If "the lineman, next to the tackle, but inside the WR on the line (who is NOT A "TIGHT END"), legally shifts to become a "Back", and happens to be wearing an eligible pass reception number, he would be eligible to go wherever he wants prior to, or during a forward pass.

Accurately knowing who players on the field actually are, as defined in NFHS 2-32 can be really helpful in understanding what they are permitted to do.

What about NFL rules?