He could have just pretended to be an AT-ST driver wearing the smuggler vest as a trophy (like the Stormtrooper who inexplicably holds Han's own gun on him instead of his officially issued blaster, lol.)
Yep.
I always thought that guy had had his Imperial blaster taken off him earlier by the rebels and then was like "I'll just take this rebel guy's cool gun then." Pragmatically it was also done so that Han could believably and quickly get his own blaster back to use it in the forest battle - the guy with Han's pistol "just so happens" to be pointing it at Han up-close when the Ewoks attack.
It's a mixed up sequence for sure - rebel dressed up as a biker scout, a stormie using Han's pistol (a stormie also has Chewy's rifle) and then Han returning the favor to dress up as an ATST driver.
And re: lowering the throne height, it almost appears that the HT throne actually needs to be raised slightly, not lowered, to make it screen accurate (the Sideshow throne is maybe 3/16" higher off the floor than the HT.) This pic also shows how HT's paint app is a bit too black vs a more sliver/gunmetal finish - the SSC throne has a more silvery/gunmetal look versus the HT that reads as black with only a slight metallic sheen. As I mentioned earlier, a more slivery look might have contrasted better with the black robes as you can see in the screen pic (I've obviously lightened the screen pic to make it more visiible though.)