[Why do I keep conceding goals from through balls?]

I have the same problem, and tested it in Fantasy Draft Mode to nullify tactic knowledge modification. 

And I force them to regroup

But I started thread maybe in wrong category

 

 

[Low defensive line setting is not working, defense is very high anyway]

I don't know how to make my defensive line drop back when out of possession

Very defensive mentality, Low block, Much lower defense line, Drop off more

Tested in Draft mode
No opposition instructions