Say, curiously, since we can combine Poison and Armor Piercing to grant one damage to each opponent...so I wonder about the Force Blast - can use that also to grant one damage?

No, because Armor piercing doesn't neccessarily GRANT one damage. It merely negates all forms of reducing it below 1. It doesn't add any damage at all. So since a Force Blast is 0 damage, Armor piercing would be no help at all.

Actually, Force Blast does NO damage, so there will never be any damage to reduce to one.

I'm thinking any walls knocked into would deal the damage, not the character, so no armor piercing damage from a wall collision.

Yeah, you would be right. That would be knockback damage, not damage dealt from a character, so AP doesn't apply to that.

To put it in another context -

When you hit someone and do knockback they take their damage, get knocked back, and then apply damage if they hit something. Since there are two separate damage sources the character gets to apply their damage reducers (if any) to both sources of damage.

Now say the person hitting has Armor Piercing. The Armor Piercing would apply to the first hit (the one causing knockback), but not to hitting the wall.

Now say the person hitting is using Force Blast. Since Force Blast is a no-damage attack there is no damage for Armor Piercing to apply to.