5 Nights/4 Days in Netherlands in December

I can't answer your questions but I'll reply to bump this back up because I also happen to be in Amsterdam the same time as you and am interested in learning more about Lichtjesavond.

Do you know about making an appointment for security clearance at Schiphol? https://www.schiphol.nl/en/timeslots/

I can't comment on Lichtjesavond, but in general, there is not much reason to stay in both Delft and Amsterdam as they are so close by frequent train service. It's much better I think to stay in one hotel and not have to worry about checking out and checking into another one. The big caveat to this is to keep in mind that hotels in Amsterdam can be very expensive.

It is difficult to answer the question about Leiden/Haarlem without knowing more about what you want to do. Delft is cute but there isn't a whole lot going on. I did the RS walking tour, the Frans Hals Musuem, and visited the New Kirke and that took one busy morning. So, in theory you can combine Delft with a stop in another city, but that depends on what you want to do in those places and how long that takes. Haarlem in particular is so close to Amsterdam that you can easily visit it as part of your Amsterdam days itinerary. Leiden was probably my favourite Dutch city to wander around, while Haarlem has the best church to visit in the Grote Kerk (excuse my poor Dutch spelling). If you enjoy art, then I recommend combining a visit to Delft with visiting the Mauritshuis in Den Haag.

I wouldn't take the time it takes to check in/out of hotels for a one night stay in Delft. I'd just take the train in for the day.You are already planning on moving hotels while in Amsterdam? I'd just choose 1 hotel for the entire 5 nights. You are spending a lot of precious time with the hotel changes. And charming Haarlem is basically Amsterdam suburb. So just visit while you are in Amsterdam, not on your way to/from Delft.