1) PowerPC accelerator cards for A1200/A4000 have _both_ the 680x0 chip and a PPC chip attached. The software you need to load does what is called context switching between the two when you're using the PPC because the AmigaOS software in its OS 3.9 and lower version cannot run natively on the PPC. So, that runs on the 680x0 chip and your math intensive stuff, like datatypes for decoding images, can run on the PPC. Yes, PPC chips are used by Macs.
2) A1200 can support PCI cards, but you have to put the machine in a tower and add what's called a Mediator board to it. The PCI cards plug into that. Depending on the accelerator card you get, that will determine the amount of memory you can have. The machine can easily support 128 MB RAM, or more if you have a card that can take more.
3) Backward compatibility with older versions of the OS is sketchy, especially if you have a PPC. Typically the PPC software needs to have the most recent version of the OS installed.
4) IBM also manufatures PPC chips, but I don't think any of those are used for classic Amiga or AmigaOne systems (could be mistaken?). In my opinion, the OS makes it an Amiga. Doesn't matter what hardware it's running on because that's transparent to the user when it's running. I believe that the A1200 port on the A1 you mention has been eliminated from the design, not sure.
