I really hesitate to respond to threads like this one, but don't want you to feel ignored. Basically, you're asking us to possibly second-guess someone who should be a professional based on second- and possibly third- hand information posted by someone who admittedly knows "nothing about a car." That's dangerous territory. I will say though, for claiming to not know anything about a car, you do a good job of talking the talk.
I will say that if a car goes in to the shop for what seems to be an obvious misfire, if the spark plugs and/or spark plug wires appear old and due to be replaced, I cannot imagine any shop performing a compression test or cylinder leak down test unless they have reason to suspect more serious damage. This could be indicated by an abnormally clean spark plug from one or more cylinders, or possibly a spark plug with a slightly green tint, or a spark plug that is wet with fuel or heavily carboned up. Unless something abnormal shows up, it's unreasonable to think the shop would perform additional tests before performing a tune up. And since it sounds like the car was running well after the tune up, again I don't really see fault in what the shop did at that time.
Now, according to the information you provided, the subsequent diagnosis is suspect. First off, 80 psi of compression seems quite low for what the shop seemingly considered okay compression. I'd be more prone to suspect they didn't have one of their spark plug wire connectors securely connected, causing the misfire after you were part way home. But, to further test the engine out, there are a few tests in addition to the compression and/or leak down tests that could be performed. First, if it has low compression, that compression has to be going somewhere. They could test for combustion gasses in the cooling system. This is a pretty simple, quite common test that doesn't actually require expensive diagnostic equipment but will indicate possible head gasket damage. The compression could also be due to rings that are not sealing well against the cylinder(s). This would often result in either burning oil (possible blue exhaust or using oil with no apparent leak) or gas flowing past the rings and contaminating the oil. In the latter case you should be able to smell the gasoline-rich oil unless you do a lot of highway driving. You may also notice an over-full condition on the oil dipstick. You could also have a loss of compression due to valves that are not properly sealing. The cylinder leak down test would identify this as an air hiss either in the exhaust or in the intake system when the tester is connected and cylinder is at top dead center.
-Rod