Hello...Zen Cart 1.5.4 - upgraded 1.3.x (can't remember)

We are using the Stock by Attribute and all seems goods - works as intended. BUT one situation is our inventory - when out could be out forever - so there is no back order.

1. So if customer A puts item size 12 in cart and checks out - item size 12 is no longer available to see at site - perfect -exactly as should be (thanks...good plug-in).

2. BUT if customer A puts item size 12 ins cart and customer B puts item size 12 in cart - this is also good - as we don't want inventory to change just because it's in a cart - but rather that it's been checkout then it's gone.

3. BUT if customer A checkouts and there is only one size 12 customer B can also check out - What I want is customer B to get a message at cart and not be able to check out if customer A already did. We were able to set this up for general inventory - it can be in two carts but only one can checkout - but couldn't get to this to work at the attributes level.

Questions:
1) Is this possible? (two attributes in cart but only one can checkout if attribute inventory is gone after that first checkout)
2) Anyone else solved this issue? Simple fix that you can describe here - or very complex that you hired someone to fix (and they actually fixed it)?

Thanks so much.