You don't need a PESEL - as far as I know, I've never had one. If however you want one, you can get one in Czestochowa.
A) He hasn't declared he is renting it out (tax avoidance)
B) Once you register at that address it would be almost impossible for him to evict you legally
It is possible to give someone a temporary zameldowanie, and in any case point B is incorrect - except in exceptional circumstances (usually involving children) the owner of a property can cancel a tenant's zameldowanie whenever they like. Tax and utilities are the usual reason for a landlord dragging their heels.