This is a cycloidal pump https://en.wikipedia.org/wiki/Cycloid not much unlike gerotor https://en.wikipedia.org/wiki/Gerotor.

Parts should fit smoothly, otherwise get your sandpaper out of the drawer.

Use 16 pcs M3x15 machine bolts for assembly, pref. stainless.
As axel, I've glued a hexbitholder to the inner rotor. Press fit might do it though.

No

No

0,1 or 0,2 mm

20%

Print with accurate setting e.g. "Engineering". Avoid oblong parts by making sure your printer is accurate in the XY-directions.
Use 3 walls/perimeters

Might require some scraping or sanding to reach smooth operation. If you feel it snag when turning, it's properly a z-seam that needs to be removed.

For super smooth operation, pump some toothpaste-water slurry in a small suitable container.