Can I make a table of all items only in a specific layer?

I don't mean going through all items and testing what layer each is in, but if I can take a layer, let's say layer "selectable", and go through all the object in there, bypassing all others, for speed and ease.

If that can't be done, then the first option is viable, and I've already got that done.

If you mean is there a GameObject.FindGameObjectsInLayer function, no. Any internal function would still have to go through them all and return the ones you specify (this is why the Find functions are slow), so if you've already written a replacement function that does this, well then there you go.