ADSL with a MK2 BT faceplate

[#1]

if a house has a MK2 BT faceplate but has ADSL rather than fibre, and has extensions running from the master MK2 socket (wired internally to the master socket), are those extensions pre-filtered?
reason i am asking, a house has this setup, but wants to have the ADSL router connected to a different socket in the house.

[#2]

Yes, the extensions will be pre-filtered.
You would have to move the extension wiring onto the AB terminals and use filters everywhere.

[#3]

and can they wired to the AB terminals by joe public? (ie i mean are you allowed to do that).

[#4]

To the AB terminals on the VDSL face plate yes, on the back plate no.