A player on team A is called for a T. Can team B sub a player in to shoot the foul shots?

Yes.

B can even sub B6 in for the first and B7 in for the 2nd, then pull them both back out before the throwin (within other substitution rules).

Just to be sure I understand the "other substitution rules"

Team A is charged with a single T. Players B1 -B5 are in the game at the time the T is called. B6 replaces B1 to shoot FT 1, then B7 replaces B2 to shoot FT 2. It's true that B6 and B7 could leave the game before the ensuing throwin, but B1 and B2 cannot re-enter until the clock has legally started. So, I guess B8 and B9 would have to come in.

Is that correct?

Correct, subs may enter and leave, but all players leaving the game cannot return until the next opportunity to sub after the clock runs.

True in NFHS. Not true in NCAA---both FTs must be attempted b y the same player.