IG on Intercepted pass
 

3/10 at the A-2. A1 drops back to pass and under a heavy rush and duress the throws the ball (a) over his head where it hits ineligible lineman A50 on head and bounds into the air (ungrounded), or (b) 20 yards downfield to what he thought was a vacated area. In both cases, the pass is intercepted by B1 and returned for a touchdown. By rule what do we have and how are we REALLY going to handle each situation?

In both cases, you'll have B1 and company celebrating a pick six.

The interception has nothing to do with the IG call. In this case it's simply a foul by the offense prior to a change of possession that produced a score for the defense. B's option would be to accept the penlty for IG (5 yards, loos of down, from the spot of the pass, 4th down for A) or decline the penalty and accept the results of the play, which would be their TD.

Actually, under the NFHS code, the foul is called an Illegal forward Pass, of which there are 5 seperate categories. Two of which are commonly referred to as "Intentional Grounding" are in fact spelled out under NF: 7-5-2 as:

(c) A pass intentionall thrown into an area not occupied by an eligible offensive receiver. and
(d) A pass intentionally thrown incomplete to save loss of yardage or to conserve time.

There's no reason to flag either play, especially b. If in A, the lineman is well behind the line and the QB is in the process of being tackled (not just harassed), then perhaps. But still, doing something unwise that results in the other team's score doesn't exactly fall under gaining an advantage.

The key word in the NFHS description of either of these Illegal Passes is, "intentionally", which removes the action from being, "something unwise" and makes it a deliberate attempt to negate an advantage the defense has gained honestly.

Yes, that is a judgment of the players intent, that a Referee is charged with making.