My Multiple Entry Non-Immigrant 'B' Visa has expired, but just before the expiration date I did a border run and got the 90 day extension. I am now in the extension period.

Next month I will go to my home country and get a new Visa for another one year period (I will do that before the expiration of the 90 day extension).

But what happens if I wish to leave Thailand for a short trip and come back, BEFORE I go back to my home country and get the new Visa?

I assume I cannot re-enter under the expired Visa.

And what about my work permit? It is good until November, but as I understand it is only good as long as my Work Visa is current.

You are not on an extension - you are on a normal 90 day visa entry. You can obtain a re-entry permit to keep that period alive if you want to take a trip and return before the expiration date but you will get the same expiration date you currently have when you return.

Your work permit is not tied to your visa - to legally work you require both a work permit and non immigrant visa entry so if you came back on a tourist visa you would not be allowed to work regardless of having that work permit - but the work permit would still be good and usable once you obtained the non immigrant visa entry.