KL/AF employees are indeed stickler's for rules and the 'that is not possible' is their way of saying that something is against the rules. I have never experienced them making up their own rules as someone else suggested. BTW, this is not KL/AF specific but is the general culture throughout Europe almost wherever you go though folks in Italy & Spain are less of sticklers for rules.

DL's system does not forward FF #/status to AF/KL as NW did. Hopefully they will fix this as it's a real PITA. If your BP included your status they recognized that without problems but if it doesn't have it on there you'll need your plastic for any perks.

Most intra-EU flights do not have seat assignments prior to gate. If you want a good seat get to the gate early. Most KL GA's are more than happy to try to get you what you want unless they think you're a prick.

ANY seat you are able to pre-select is only a request, not a guarantee. Kind of sucks, but that's the way they do it. If you select a Y+ or similar that costs extra expect to be moved out of it. If you want it get to the gate early and make sure they'll give it to you. Hopefully DL will add these capabilities to their website at some point so that probability of getting preferred seats is higher.