To answer your question(s):
The penalty is assessed if the ineligible player is more than 1 yard down field on a forward pass play unless he is blocking a player that initiated within the 1 yard buffer zone.
If the defensive player breaks the contact with the ineligible player, the player must stop. He may not continue down field until the pass in completed.
Once a pass is complete all players may advance at their own discretion.
In the NFL there is no "screen pass exception", this is a NCAA rule where a player can be down field if the pass in completed behind the line of scrimmage.
NFL - Eligible Receiver
In order to catch a forward pass, a player must be an eligible receiver:They can line up in the backfield (ie running back, quarterback, or receiver) They can be on either end of the line of scrimmage (ie a tight end or split end)
Also in the NFL an eligible receiver cannot wear a number of 50-79 or 90-99 with more...