You can generally re-sell gig tickets, at an appropriate price.Better than having the seat empty for such a big game, surely?
And I don't think you can promise refunds if people can't go, or everybody in category 1 would just buy a ticket and then decide later if they can actually make it to Amsterdam. That route would be chaos, a ticket could end up being sold and resold multiple times. I don't know if these were etickets or paper tickets, but either way it's a lot of admin. Meanwhile the people ìn cat 2 who eventually get the tickets would face increased plane or hotel fares, or just no availability. That's not fair on them.
When you buy concert tickets you can purchase insurance to get a refund if you can't go for heath reasons, I don't know if this is offered with these tickets, or whether clubs regularly in Europe do this, but it could be a way to go in future.
Selling a ticket twice, without reimbursing the other individual is wrong. In this instance it was a potentially life threatening illness.