Endpoints (x₁,y₁) and (x₂,y₂)—midpoint coordinates.
Yes—((x₁+x₂)/2, (y₁+y₂)/2) with each coordinate shown to six decimals. Any finite numbers are accepted; commas are treated as decimal separators like the slope tool.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.