Docs in openHAB3 are not ready yet, one reason is it is not released (still a milestone).
My intuition tells me that whatever I select to expose here will be exposed to myopenhab.org (and hence Alexa) and everything else will not be exposed.
You don’t need to expose any items to the cloud for Alexa to work. Due to server abuse this function is disabled anyway.
Correct.
Correct. But you won’t find those instructions in each and every binding section as this would be repeated then a couple of hundred times. You should find that in the general doc section.
There are only a few users yet on openHAB3. I suggest to start reading the wiki posts: