sorry to hear about your troubles of migration. A while back (2016) I have migrated EventList to JEM - had to go table by table and map existing fields and update new ones.
Your error is related to duplicate key - for event 682 you have category 3 assigned more than once (catid-itemid only expected to appear once, must be unique combination)
Good idea to sort the ordering of event categories, but that should not produce any errors.
So, old EventList had 7 tables, and JEM has 10 tables
And in jem_events are 96 colons, but in old cal was 26 :-D
But I opened all of that in excell and restructured like new tables and put into JEM through Administration (Impoet csv) and it works great now.
I had problem with events which are repetiting, because JEM maked new events all the time, and I had error but I solved it with put all 0 in 3 columns so no repetitions for old events now. and no error or problems