>the hive has virtually no brood left (only a few capped cells),

This is normal. They were hatching, getting capped and emerging while the queen is being raised and none are being layed.

>they have been building a little bit of comb from foundation and there is one capped queen cell as far as i can see, but I may have missed some queen cells.

They are easy to miss since emergency cells are seldom down low and all queen cells are so covered with bees all the time that they are hard to spot.

>They are building another one in the middle of a frame but it is not capped yet and I don't think I see a larva.

I'm sure there isn't a larvae. They just felt like building a cell cup.

>Since there are no eggs there and I am uncertain about the existence of capped queen cells,

I thought you saw one?

>I wonder what happened. I expected several queen cells capped and getting close to hatching by now.

Sometimes there are several sometimes one or two. Usually two or three for a small nuc is about normal.

>I will try to order a queen, but should I? I would have loved to see a queen be born, mate, etc. but I may end up reaching the end of the season with a hive that is far too weak.

My guess is, if you did see a sealed queen cell, that it is about to emerge. I didn't keep track of exact days, but it takes 16 days for a queen to emerge. And another week or so before she starts to lay.