I did something similar 2yrs ago.

Note that if you ahieve E50 as a gift, challenge, etc, all the counters still start at zero. It just makes it look pretty bleak whem amount needed to 'next tier' E75k is a full 75k away (less 18k in your case for completing a challenge). You also will likely get the E50k Eupgrade credits ahead of time, meaning no more are awarded when you do get the meter past the E50k AQD & AQM

When I contacted AC they would not consider a challenge to another *A carrier per their *A agreements. They said (similar to what United website says) if I held *A with any other airline they system would be unable to proceed. At the time they would accept Oneworld or Skyteam high tier status but had said to include proof of flight history (to avoid piggybacking off of a different carrier who may have awarded or matched status).