Downstream Margin: 14 db
Downstream Line Attenuation: 29

the margin is a bit modest for the attenuation at 2M.

Please describe the wiring in full.

Why is the Master socket 'crappy'?

Replacing the Master socket with a NTE5A is not a panacea for all ADSL problems.
It can help where internal extension wiring is problematic, as this can then be isolated with a faceplate filter, but if you only have a single extension in series with the Master, you are unlikely to see any improvement.

Assuming BT Retail is your telephone provider, the BT Price List says (at the bottom of the page) that you can pay �25 to convert from a Master socket to a NTE5.

"I would have expected them to have fitted the split face plate"
At a guess , I would imagine they (Openreach) replaced the overhead drop wires, possibly as a result of pole replacement.That would seem the most likely reason for doing a whole street.
There may have been no requirement to change any of the internals.

Your best bet IMHO would be to remove the ring wire from terminal 3 of every socket in your house.