Don't have much help on the first two, but can help you with number three. R12 is controlled. To purchase it you need an EPA 609 certification. There are a number of websites where you can go and take an open book online test for about $15 to get the certification. You might try IMACA.ORG.

You can then buy it at NAPA or you can usually buy it online for about $15 a can.

You are correct that a low charge will interrupt the system. There is a low pressure switch in series with the clutch to protect the compressor against a low refrigerant situation. Low refrigerant will prevent lubricant circulation.

You are ABSOLUTELY correct in staying with R12. 134 will not cool as well, and statistically will shorten the life of the system. MANY people will promote an alternative refrigerant, but with R12 getting as cheap as it is, it's just not worth it. All the alternatives are either flammables or blends and there are disadvantages to both categories.

Hope this helps.