Belt keeps coming off

The description sounds like one of the accessories is binding or an idler bearing is bad. With the belt off follow it's path around the pulleys and parts and see if they all spin freely and have no play like rocking or rattling. I suspect you will find that one or more has a problem. Once that is repaired you will also want to replace the belt as they can easily be damaged internally and cause issues with broken internal belting or soft areas from chemical exposure. Something like a bad idler or compressor clutch bearing can easily throw off the belt, even if they are not loose enough to allow wobble.

You're welcome, please let us know what you find.

Well, that isn't something you see every day. If it's the entire pump moving it has either broken some of the mounting tabs or the bolts all came out, I'd suspect the tabs myself. The book shows two different pumps for that car, one that fits the vehicles with self leveling suspension and one that fits without that. But it also says to order by the code on the pump. As for replacement, they are both about the same, drain the fluid out, disconnect the lines and remove the belt and remove the pump. If it has broken free you will need to determine if it's the pump or the brackets that are the problem. For the brackets a salvage yard is likely the better option. From your description of it squealing I would say it has been working it's way loose for a while.