I agree
 

However, if they plant their feet and are displaced it is definitely a foul. If a player gives ground on their own with little contact, I've got nothing. Remember, not all contact is a foul.

We have to judge advantage/disadvantage. If the defender chooses to move when there is incidental contact, then I've got nothing. I see this more frequently at the lower level where the offensive player moves backwards and the defender moves her feet with them. Very little if any contact. The contact didn't displace the player. The defender chose to move backwards on their own. I didn't mean to imply that they have to plant their feet to get the foul, just that if they are moving backward it now becomes a judgement call whether the contact was incidental or not. But that's what we get paid for.

Agreed, it's the same as a "moving screen." If the screener is moving, but the defender voluntarily gets out of the way or moves wider to go around it, it's nothing.

Could this physical act being discussed also be same/similar to the commom scenario of player A1 "sealing" player B1 from a loose ball that is traveling towards OOB?

Exactly. If the "sealing" player gets to the spot first, it's legal. If they create contact while moving and displace or impede the opponent, it's a foul.

Unless, of course, they are moving in the same path and direction -- which is what I judge 99% of the time.

This is the key word I was looking for in the OP. If A2 is impeding B1 from coming to play defense on A1, we would have a foul. Did I interpret that right?

Good point, I was thinking of lateral movement by the "sealer." Normally, however, the sealer is either moving slowly with the opponent or gets to the spot in time.

Yes, if the impediment is caused by illegal contact for which A2 is responsible.