If you are good at vent sexing then use it only. Unless you have breeding a fast feathering breed to a slow feathering breed and the feathering speed is linked to gender, it does not work.
It does not work within a breed. You can tell with some breeds later, at 4 weeks or so but still not...
Being 1.5 degrees too low will definitely cause the problems you see. There should not be cold spots that are that far off in the incubator. Check on air flow.
Yes, move the eggs around every day or so to even out the problems caused by cold spots.
From what you are describing, the temps were too low in the incubator.
Late and wet chicky chicks are almost always from that.
Get a brinsea spot check and adjust the temperature on the incubator. The accurite from Walmart are not reliable enough to use for incubation.
From pip to zip can be more than 12 hours. Once the first one hatches, the hatch is usually over in 24 hours.
I have one breed that hatches over 3 days though so you never know. It can be even longer if you are hatching different breeds. Some breeds will be on a different schedule even when...
One day after the last one hatches, I check for signs of life.
Chirping, movement when candling and etc. If I see signs of life, I will poke a hole into the big end to see if I can get something going.
If I do not see signs of life, I open up the eggs to see if I can tell what killed them.
I understand culling for genetic issues but you are culling for feeding poor nutrition.
That seems to need more thought. On the surface it does not sound like the correct approach.
Another thing is that silkies and other breeds with vaulted skulls can have a problem with brain injury...