An inside corner tool should strike off a clean corner.
I use a 6 inch knife for everything.

I have no problem with inside corner in general - it's where it meets the ceiling that causes me fit.

1 idea I'm pursuiting is to borrow a Dremel hand craft tool (or similar handy craft tool) to finely sand this hard to reach corner down a bit.

I think the secret to corners in general is to not apply too much mud at one time and only apply it on one side at a time. If you do it right you should have no or very little sanding right in the corner. The sanding should be away from the corner to blend it into the open wall.

Oh but I forgot to add that I like to use those sanding sponges with the angled side - makes it easy to get into a corner if you have to.