The file I'm using was included in a ham call lookup app. It appears to be a very clean derivative of the FCC file and very consistent with eQSL. I just recently started concentrating on counties and have 1,020 confirmed in eQSL ,thanks in large part to JTAlert, with every QSO county agreeing with the county in the call master file.
In terms of impacting JTAlert, I imported the call file (CSV format) into Access and queries against it take about 1 second to complete so I don't think the overhead would be noticeable.
Reviewing the eQSL activity (or lack thereof) for US hams has been interesting. Using 1/1/21 as a starting point there are only about 1,600 counties listed with any activity from US hams in eQSL. This means that you must resort to paper QSL's to get beyond the 1,500 endorsement level. Given that there are over 3,000 US counties that could be an expensive and time consuming effort to get them all. The best solution would be for ARRL to implement a county function in LOTW similar to what is provided for CQ WAZ and WPX.
I always thought there were many more US hams currently using eQSL but that appears to not be the case. I don't consider a member who hasn't uploaded a log in the past 6 months an active user.
Looking forward to the next release.
73 Pat N6PAT