The winter and poor spring was difficult for bees and I found that colonies - particularly nucs - were smaller than I would expect in other years. As an example, one nuc had a patch of brood about 4" across on one frame - so it only just
got through winter. In March I inspected a few colonies - when it was too cold to do so - and found they had no pollen - so brooding had stopped.
I have had lots of enquiries from beekeepers who lost their colonies over this past winter. I suspect that recorded winter losses will be high so as The Drone says, you should not beat yourself up about your losses.
Last year I was asked to go to a colony with very few bees and a queen. I picked out the queen and popped her in a colony that was able to support brood and she laid all season; indicating that the few bees she was with were just not able to rear brood with her....
I suspect your few hundred bees is not enough to keep the brood warm for any queen that you think is OK. Combining colonies with one queen is usually an option, although a laying worker colony should not be combined.
Putting weak colonies into a small nuc is also an option as the bees then have a small space to heat and can have a chance of rearing brood. However you have a few old bees remaining and I am not sure whether it's worth continuing on with them. We are well into May - if your two colonies are close together you could move them until they are a few feet apart and then combine them with the best queen you think you have and put them in a small nuc if you have one and see what happens. There is little to lose.
Note Spring dwindle can happen due to nosema. One type leaves little tell-tale signs. The other results in dysentry which you might see on the frames or outside of the hive(s).