So then by this logic who ever it was that fixed lc needed someone to let them either past the wards and/or past the threshold (at least so they could use magic while in harry's apartment) AND was either someone Thomas trusted or could fool him into letting them in.
Oddly enough it makes Mab (or Lea) more likely than future Harry baring some special rule about time travelers and thresholds. but it also means any witch or wizard who are decent at illusions or transformation would have been capable, notable examples being Elain or Joe (or if your in need of a time travel fix future molly
). however the pool of people thomas would have trusted AND would be capable(ability and knowledge) and interested in fixing it is remarkable small.