" (and should entitle one to a partial refund or credit)."
That's going to be a matter of contract law, subject to the terms of your charter contract, subject to the laws of whatever jurisdiction that contract is subject to.
Some of what you mention is plain stupid on the charterer's part, scalds are a liability issue in the US and might very well be elsewhere. But then again, the average hardware store in the US no longer sells anti-scald valves, they were a passing phase.
I think the answer is to go into the charter with eyes open. Ask the company up front what will be guaranteed, and what compensation you will be given if various things (engine, sail, oven, reefer) aren't working. If they have a larger newer fleet, it is possible to move you into another boat. If they're just managing other people's boats (OPBs) and the boats are somewhat older...sometimes you get what you pay for.