I am shown the following screen when I go to withdraw my ADA from a delegation.
It reads as if I deregister I will stop receiving rewards. Does that apply to the last few epochs where rewards have been earned, but not received as well?
Yes, it does. At the moment you deregister your stake key, you will get your deposit back and at that moment, you are no longer able to receive rewards. All your rewards will go to the treasury.
When you delegate for the first time, only 2 Ada is "locked up." The rest of your Ada can be spent at any time you choose. So if you're simply wanting to "cash out," you do not have to deregister to access your money.
If you decide your time in the Cardano community is complete and you truly want to divorce yourself of alllll your Ada under the sun, then you may deregister, and get that 2 Ada back as well.
With rewards payments lagging by two epochs, your best path is to
No, you will not receive rewards if your stake is deregistered.