Can I move a unit into an occupied battlefield with standard movement (tap) and move another unit with Sett's signature spell (Showstopper) into the same battlefield before the start of the showdown?

Ruling: No, you cannot combine standard movement with non-standard movement like Showstopper in this way.

Nuances:

• Showstopper is a slow spell, not an action
• If it were a fast spell like Zenith Blade, you could move your unit with standard movement, then during the showdown use the fast spell to move another unit onto the battlefield