Hello, tintinfrost! Thank you for your question, I'll do my best to clear up any confusion. c:
First of all, the limits do not actually apply until after you clear the map. So using your example:
Unit A and Unit B kill 6 enemies, gaining 36 points and reaching their limit with each other. They now have 22 points left. However, if they gain more than 22 points with Unit C, say 28 points, Units A+B will not gain 36 points with each other, but 30. Then those 30 points are rounded back to 36. Units A+C keep their 28 points, but then they are rounded down to 24.
Simplified:
A+B gained 36 points and kept 30 which were rounded to 36.
A+C gained 28 points and kept 28 which were rounded to 24.
We can also add Unit D with Unit A, let's say they gained 14 points. In this case, Units A+C will not keep their 28 points, but only 18. Since those 18 points are still rounded to 24, in the end it makes no difference.
Simplified:
A+B gained 36 points and kept 30 which were rounded to 36.
A+C gained 28 points and kept 18 which were rounded to 24.
A+D gained 14 points and kept 10 which were rounded to 12.
So although Unit A earned 78 points total, only 58 of those points actually counted, but because of how those 58 points were rounded, in the end 72 points were gained.
As for the scenario where you have earned 36 points in one map, and then gain 6 points on the next, that certainly had me puzzled for a while. But it's how the points are rounded, again. If you gain anything less than 8 points, you gain nothing. So those 6 points, even though they should be enough to put you at 42, are being rounded right back down to 0 before being added to your 36 points.
And you are correct, aside from using Seeds of Trust there is no way to unlock B to S supports in a single map. Knowing how the points are being divided can help you be more effective about it nonetheless. Since you only need to gather 58 points to get 36+24+12, if you know what you're doing each unit can gain an additional 14 points for free once they're rounded. ^.~
Hope this helps!