I'm going to be exporting a DVD from premiere to watch on a TV. Should I use a field order of upper or lower fields first, or does that not really matter? Should I rather deinterlace/set it to progressive scan?

if ur source is interlaced, stick with interlaced... if its a dv source, use lower field first

hdv upper field depending on ur camera

What would be the difference between selecting Progressive in the field order menu and simply checking the deinterlace box in the top left of the settings? For already progressive footage, will Premiere detect this and blank out the field order/deinterlace option?

progressive scan within DV is seen as interlaced (as the interlace stream is the transport stream of the progresssicve frames),
In post, you use a pulldown service to remove the exceesive frames.

if its native progressive, its recomended to render as progressive, as pulldown add/removal will usually be autoamtic anyway, based on the output files requirements