Yes, “www” and “dev” are two completely separate FQDNs.
Yes, all changes (adds and removes) will need to generate a new cert.
Only “wild card” certs can overcome the addition of yet unknown names.
FYI: Wild Card certs are coming to LE in January 2018.
The question is rather confusing…
Server A has no certs - so, yes, you will have to obtain a cert to use a cert.
And the FQDN must be resolvable via global DNS.
In general, yes.
But this depends on the auth challenge methods used and even the IP to which the names resolve.
If they all resolve to the same IP, then you could generate all the certs (or one cert with all the names in it) from one system/server.