IMHO, if the WR has departed his position on the LOS, then the "eligible numbered tackle" becomes uncovered and is, thereby an eligible receiver. I don't think the rules specify that a player's position is dictated by where he was last "legal"; or, that a "covered receiver" remains covered once the "covering" player vacates that spot.
Did I actually agree with UpstateAl?
A offensive player in the game can be one of three things: a back, a lineman, or
neither.
If the WR never established himself as a BACK,
by stopping for a second in the backfield and without breaking the waistline of the snapper ,
Team A has a different problem: Illegal Position at the Snap - since he, the WR, is now neither a lineman nor a back.