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Hello and thank you for trusting us with your question. I'm DJ.
Sounds like the mechanic may be just guessing at what the problem is.
To see if the fan belt needs to be replaces, spray Windex or water onto the belt when it's squealing. If the belt is glazed and causing a squeal, the noise will be reduced immediately. Then the sound will start again as the Windex dries off. Don't use anything with oil like WD40. That will just make the problem worse.
I need to know more about the overheating problem. Does it overheat more when you are on the freeway or climbing a hill? Or is it more likely to overheat when driving in stop and go traffic?
The oil leak probably has nothing to do with the overheating problem unless oil is getting onto the drive belt. This could cause it to slip. Hard to tell from the picture, but it doesn't look that bad.
Let me know what you can about the overheating and we'll figure this out.
Does the vehicle leak coolant? If the radiator housing is cracked, it would be spraying coolant out.
The leaks have to be fixed before you can really condemn anything else on the vehicle.