No-one sensible cares a thing about whether someone's gay or whether someone else reads (or watches) adult porn, so all that should be put aside. Let's take the real issues one by one.
Landlord / tenant laws vary from state to state, so if you cannot reach agreement in this forum, you should discuss the specifics with a lawyer familiar with the laws of PA.
Rent: The tenants signed a contract to pay rent for one year. The landlord has a right to expect that the rent be paid and most landlord/tenant laws and leases specify that the entire year's rent is payable, even if the tenant moves out early, unless arrangements are made for early termination of the lease. Tenants should recognize that landlords have actual costs to owning a property and loss of rent is a serious issue.
The tenant admits to owing $900; is this correct? If eviction was filed, wouldn't it have been more than that, even just for time actually lived in the house?
Without having all the facts, I'd say that at a minimum the tenant should pay the $900, plus rent for any other time occupying the house without rent payment, plus at least one month's rent. Even in a highly desirable rental area, it generally takes one month to prepare a house and find a new tenant.
Lock changes: It's rational for the tenant to want the locks changed after a previous tenant (drug-addicted or not) may have retained keys; however, this should be allowed for in the lease or negotiated. Further, when a tenant makes unauthorized changes in a property, they must restore the property to its original state. Say the tenant painted a white-painted bedroom navy blue without permission - they'd have to repaint it to the original color before leaving, or pay for it to be done by the landlord out of the security deposit. The tenant should have made arrangements with the landlord to get the locks changed but failing that, should have reinstalled the original locks. I would also be throroughly upset a tenant left the house unlocked with the locks on the floor.
The landlord has the right to have a copy of the key for many reasons, including emergencies. A landlord has to give the tenant notice before coming by the house, EXCEPT in an emergency, and all the tenant would have had to do would be to put a cheap security chain on the door if unexpected entry was really the issue. In the event that the landlord was really making unauthorized entries or otherwise disturbing the tenants' "quiet enjoyment" of the property, a police report should have been filed or written notice given. Otherwise, it's just he said/she said.
Leak / furniture: Tenants are supposed to carry renter's insurance if they want their property covered. If the tenant informed the landlord in writing that the pipes were leaking in, say, Sept., and he didn't show up to fix them until December, the tenant might have had a case. Burst pipes don't fit that scenario at all, however. Also, if the landlord did not have a key to the property (per both parties' accounts) then that would or could have delayed necessary repairs.
It is a mystery to me how the tenant could have had $1,800 in brand-new furniture but couldn't (by her own admission) pay rent.
Toilets unflushed - There is a big difference between just being gross and actually vandalizing a toilet. If the toilet was blocked with refuse etc. and a plumber was needed, that's one thing - and the plumbing fee should be deducted from any security deposit or added to the total owed. If the toilet was just unflushed, that shouldn't be a big deal. If there was refuse in it, or it was exceptionally dirty, then that's a cleaning fee. Landlord should attach pictures/receipts if this item is to be negotiated.
The landlord doesn't mention this, but most landlords charge for removal of furniture left by tenants.
I would recommend that the two parties try to take deep breaths and resolve this problem. If the tenant is genuinely hard up, perhaps she could pay the $1,500 in installments and the claim could be un-posted until all payments are made.