There was no ground crew strike on 29 April that I am aware of - a wildcat strike on Saturday 23 April did, however, bring the airport into a state of paralysis, and which exacerbated the problems that Schiphol experienced right through to today. Flights have been cancelled almost daily since late April to the present, and there have been many cases of passengers missing flights despite reporting to the airport 3 or more hours in advance due to the horrific delays caused by an understaffed Schiphol unable to cope with the planned number of passengers. The airport even took the novel strategy of directing airlines to cancel planned flights, and to stop selling any more tickets on services during the periods of greatest pressure.

According to the free flight check on https://www.euclaim.co.uk/free-flight-check#/problem, they seem to be willing to pursue cases involving KL1273 on 29 April.

I would suggest you turn the case over to the claims agency of your choice; once you fill in the initial details and provide a power of attorney, they will (in theory anyway) chase up the case on your behalf. If you were successful, they would take a cut of the compensation. If they can't obtain compensation, you would not have to pay the agency (but check their terms before engaging them. I don't know of any agency that doesn't work on a "No Win No Fee" basis but that doesn't mean there may not be some who do operate on a different basis)

One final question - have you already been reimbursed for your cancelled trip? I assume so, given that you don't address this issue, which would seem to be the more pressing issue here.

I suspect that, having requested a reimbursement, which is either in the system for processing or which already has been processed, the "normal" website flow of claiming compensation is closed off, because they presumably don't expect that flights that a passenger has requested full reimbursement for should also be eligible for payment of compensation. Or, maybe it is just as simple as that, having marked the case as "closed", it cannot be reopened by the passenger in the usual manner.

Again, another reason to just pass the details on to a specialist, and forget about it (because these things take time).