>>> (b) How can I turn OFF the "auto-complete" function on the phone when starting to type in an email address ?
Um, sorry, there is no setting for this in Aqua

But if you purge the addresses, then there will be nothing to auto-complete from.
>>> I do NOT want the phone to "remember" previous typed addresses? How can I PURGE (delete) that memory-resident list of previous email addresses used on the phone? (They are NOT found in my contacts--obviously some sort of memory cache)
"memory-resident list of previous email addresses used on the phone"

Aqua doesn't keep one, if you click "recents" in the app's own contact picker, it actually scans your recent messages right then and there. There is no cache.
As for how to purge your already synced contacts after turning off Gmail contact sync -- sorry, I don't know.