my SM-T810 won't install stuff from the Google store

As soon as I got my 810, I let it update to Marshmallow (factory image)

Now I am trying to load on a number of my usual apps, and they refuse to install. I have no idea why. One is VLC- so I can watch videos. Another is all of Microsoft's Office stuff (PowerPoint, Excel, Word, etc.) Does anyone have an idea about why that might be? Or a method of debugging the store?

SM-T810 stuck in "D/L- do not turn off target." Flash stock rom to fix?

On my rooted T810 I mistakenly hit restart through Knox and ended up in Odin mode. If I do Power + Volume Down it'll show the Samsung logo for a second, then immediately go to "Downloading...do not turn off target." Other key combos like recovery mode don't seem to work for me (unless I'm doing them wrong). What should be my plan from here? I'd like to unroot at this point, so try to flash a stock rom through Odin?

Thanks for the help.