If the chain slips while staying on the same sprocket, chain or cassette may be worn out.
But I'd suggest getting the huge cabling error sorted out first, to see how much that influences things.
I'm a bit concerned about the cable "cost too much to repair at the shop".
A simple cable replacement shouldn't be much.
If you can't afford that, it seriously limits the kind of repairs you can do.
Even basic parts cost some money.