The change fee is per "one set of changes". This can be up to 15 flights for 1 change fee, but have to do it all at the same time (so if you change a flight the next day, that is another change fee).

You might consider JFK-SJU-LAX-LHR on AA as an alternative. Depending on your travel dates it may work well (but only 1 transcon allowed, so SEA-JFK would not be permitted along with SJU-LAX). I haven't checked whether HNL-DFW still operates (expect it does), if so you could go HNL-DFW-SFO-STL-JFK-SJU-LAX-LHR. Another alternative is a loop down to central america (LAX-SJO-LAX) but that changes your trip substantially.