So what is the actual reason balance fixes from kingdoms can't be added to the grand campaign? (a bit of technical detail wouldn't go amiss) As nothing stated so far seems to add up. For instance if your able to make a compatibility patch there doesn't seem to be any reason why you can't transfer over the balance fixes .
Well, we've said ourselves that the Grand Campaign is huge, with tons of replay value.

Currently, with no changes to it in Kingdoms, it does not need to be re-tested.

The new Kingdoms mini-campaigns do, so chucking in a few fixes can be done without much disruption, due to the fact that there are already testers in place.

To start up testing for the GC would require a lot more work, and they've just decided that it's not worth it at the moment, it seems.