In Schottler, I get a callout telling me to go to the Hardware Store for domestic violence. When I get there, I can't see it but I can hear fighting. Where do I need to go?

Not sure which one you mean, but make sure to check all the alleys and even the roofs. 

Use the Trainer to go in the Door at the Place that the Very Second

Mission is at.

 

Sergeanto!

In GTA IV the door to the hardware store does not open; you have to use SNT's "GO THROUGH DOORS" command to get in and out of the building.  Funnily enough the door is unlocked and works in EFLC.