Yes. Gridfinity Forge uses the published Gridfinity dimensions: 42 mm grid, 0.5 mm bin clearance, 7 mm height units, and the standard foot, baseplate and stacking-lip profiles. The tests check that a bin seats in a baseplate and stacks on another bin without colliding.

Set Model units to 1 unit = 1 mm (the default). With Scene units on Blender's default metric scene, 42 mm becomes 0.042 m, and slicers read that as 0.042 mm.

Yes. Choose Fit drawer and enter the drawer's inside width and depth. You get the most whole 42 mm cells that fit, centred, with a solid frame filling the rest (at least 0.5 mm per side). The panel shows the result before you click.

No. Print bins upright: the feet, magnet pockets, stacking lip and label shelf are designed to print without supports on a normal FDM printer.

Yes. It is a normal mesh object. You can cut it, add text or boolean in your own shapes, and Ctrl+Z undoes the creation.