Try to keep the temp a little more stable. You want the least amount of variation as possible. I keep my thermostat set to 88.5º but it sometimes dips down to 87º
That is correct. Your eggs will tell you if the humidity is too low. They should be plump, not wrinkly. A week or two before they hatch, they will start to look deflated and wrinkly, that is completely normal.
You don't want more than a few 10ths of a degree temp fluctuation for the most part. I keep my thermostat right around 89 degrees and usually have a slight fluctuation between 88.7 and 89.2. I see the fluctuation mainly after opening the inc to check the eggs and wipe off any excess condensation once per week. Your humidity is fine.