Ah.... cat burglary
Well if he's too mean to invest in a microchip cat flap, he's kind of inviting all and sundry - feline and criminal into his house.
Is Salem neutered? Am sure he is, but with him being so territorial, I thought I'd ask. Usually, neutered cats work out ways to share territory relatively squabble free.
The only time I've ever had problems with cats gatecrashing our property has been from complete toms, hence the acquisition of a microchip cat flap. Job done. Now the worst we get is a squirt on the perspex and a pungent odour when the door opens.
On another point entirely, given Salem is prone to atacking other cats, do you have indemnity insurance as part of his cover - if he injured another cat badly, you wouldnt want to be left in a position where you had a hefty vet's bill to face - or indeed if he bites off more than he can chew one day. Forgive me if these are things you already have covered - just thinking aloud, as it were.