Q:

In "nicht nur ... sondern" construction, which part should a verb agree with?

... dass nicht nur Felix, sondern jeder einzelne Einwohner erschossen worden ist/sind?

I wonder if the verb "sein" at the end should agree with the singular "Felix" or the singular "jeder einzelne Einwohner"? Or should I add these two parts together in a manner of speaking and use the plural form sind?

A:

In your example, as others answered, "ist" is totally correct, as it matches both parts.
If you slightly adjust your example, you would usually have to match the second part, although this may sound weird in some cases:

... dass nicht nur Felix, sondern alle Einwohner erschossen worden sind.
... dass nicht nur alle Einwohner, sondern auch das Maskottchen erschossen worden ist.

Also see this Link (in german), where also other cases than "nicht nur... sondern auch" are explained.

