For example, let's say B is getting curbstomped and A wants to slot fill B.
A could hit B with less troops on an unwinnable war, do like 28 resistance damage in total. B would fortify and turtle, fight the other wars. A would automatically win this war and turtle B. If B gets beiged by someone else, they could peace out with A, saying that they lost too much resistance to fight that war. If B won the other wars and didn't beige, A would beige B on timeout.
This is a harder call than previous slot filling. Before this, you could plausibly disguise your slot fill as a raid at best. This gives it a wider range of use.
Maybe one amendment is to get rid of Point 3, and change Point 1 so that it has to be a 50 resistance difference or so.
Or lowering the benefits of beige as per Point 2 would make beige less appealing. Maybe like reducing military rebuy capacity while someone is in beige. But this means that first to beige loses their other wars. It's still a better alternative than no beige, and yet balanced enough to keep a small alliance from pinning a much larger one.
Legal slot filling would basically mean two "alliances" hitting each other indefinitely. Maybe a CN style peace mode is a better alternative?