PRE fixes

Beware the Jabberwock, my son!

invaluable, inflammable, inhabited.
Why is the prefix in a bit odd in these cases? If indeed it is a prefix here.

If bi is a prefix (biplane, bicycle) meaning two of something, then biscuit is two scuits ?? So that must be those ones with two round things and a flavoured filling (those trakina thingies), so just a single one then would be a monoscuit, or uniscuit?