I agree about the dry wall mounts. I've had horrible luck trying to use those things and they ended up making a big hole and they were about impossible to remove without making an even bigger hole. Fortunately, I learned this in my own condo...and I sold it "as is"
Since then, I use long screws and make sure they go into a stud, for my heavy pieces - and most of mine are heavy - like mirrors, etc.
If this LL is a jerk regarding returning security deposits, you will likely be suing him anyway. Why not go ahead and enjoy your artwork now?
In CA, a LL has to give you the ability to fix anything yourself and get your full deposit back.
You can check your state laws here:
Security Deposit Law Guide 50 States | Landlord.com
I would go ahead and hang everything. But, I also don't mind going to court. I'm just thinking, you're likely to go to court with this guy anyway, so you might as well put up your artwork and add that to the list of deposit deductions you will be suing him for in 6 months.