On the landlord thing be sure to write down times, names ,dates things happen and what happened.
evidence - take pictures of anything that might help you prove something.
save any messages on your phone or record any harassing calls if you can.

Do you watch those court shows on tv?
They are kind of good for showing what is needed as proof.
many landlord /tenet cases

Oh you might check your county health or housing dept website and find out if any laws are being broken by landlord.
Or if an inspection can be requested??

Next time mail is missing report that to post office.
I'm pretty sure tampering with mail is a federal offense.

About all I can think to do about the yucky neighbor is to totally ignore her- don't look at her , don't talk to her..., pretend she isn't even there.