Slitherine

will restore to full strength. but doesn't this require the field repair hero? what about the Karl and other special heavy artillery units? they won't accept heroes? or is another way to get them back to full strength?

Nope, you cant any longer put heroes on these kind of units. Makes them completly useless tbh.

In the PC1 Kampaign mod there are some trainings missions, where you can upgrade/repair these units afair. But still, they are useless.