Great that you got an A500 and are bringing it back to it's former glory!
I can't answer all your questions but can help point you in the right direction for some.
To identify hardware try sites like The Big Book of Amiga Hardware (
https://bigbookofamigahardware.com) and The Amiga Hardware Database (
https://amiga.resource.cx/).
For the toggle switch first see what it is wired to. My suspicions are either a Kickstart switch (swap between versions of the Kickstart ROM for comparability) or enabling/disabling RAM. If either of these don't flick it with the Amiga turned on.
There are plenty of mouse adapters produced. AmigaKit list a USB adapter (
https://amigakit.amiga.store/amiga-mouse-adapter-p-12699.html). Or just search for Amiga mouse adapter in your favourite search engine. I bought one off eBay a few years ago (
https://www.ebay.co.uk/usr/retrobargains ) and use a Compaq PS2 mouse with it. Be careful if you buy a USB to Amiga adapter because some only support the PS2 protocol so not all USB mice work with them; the AmigaKit one doesn't have this issue.
