The short answers are, no and no.Is this something I should worry about before gluing on the neck? Is sanding the heel to get rid of the gap necessary?
As long as the bottom of the heel (tenon) is fully seated in the bottom of the pocket and the neck angle is sufficient to get the lowest possible string action, it's fine. (BTW, most of the time the neck angle is best adjusted via shimming rather than sanding the heel)
That gap under the fretboard is normal even on Gibsons. It does not get glued to the top of the body or need to contact it. The sides and bottom of the tenon are the contact areas for gluing.
Making the world a better place; one guitar at a time...
I would normally just fill it and paint to a matching colour (masking around it as necessary).