Typically the only two that are critical are reflow to clean and clean to coating and it depends on what are you cleaning off and how.

As a rule, the sooner after reflow you clean a PCB, the easier it is to remove flux and other residues. After cleaning, the PCBs should be allowed to dry completely before any power is applied.

After heat testing, they just need to cool to room temperature before functional test. Unless a second round of cleaning is required after all that testing, there is no reason to wait before conformal coat.

I have not come across a wait time from soldering to cleaning under normal circumstances. Most solders appear to be amenable to functional testing of circuits almost immediately, assuming the temperature of the board is ok.

With regards to conformal coating. PCB materials soak up moisture very quickly. You can test this by taking a board and baking it overnight at 120Deg C. Weigh the board and weight it again each hour and plot the weight gain over time. You will see that within a couple of hours the boards will soak up the ambient moisture.

There are a number of factors that can impact the adhesion of the coating to the board. I happen to believe moisture is one of them. My preference is to coat a dry board. Over time moisture will ingress through a conformal coating but the better the adhesion to the board the less chance you will have delamination of the coating.