yeah, it's be mono to a single track.
First, go with the preamp idea. That's something you'll always have a use for while the stereo mic might mostly sit in a box later on when you've upgraded your gear.
Second, I don't have direct experience with the Mocro BR but I did quickly glance at a pic of it and I see there is a menu and some parameter buttons.
The way the mic/line input is labeled, it may automatically treat the input as a mono signal if a mono plug is inserted.
Whether it does or not, there's GOT to be somewhere in the menu/setup where you can take a mono input and pan it to wherever you wish.
I'd be shocked if it didn't have some way to do that so search your manual.
I'd probably try to use the guitar input for guitar and not vox because it's described as a "dedicated guitar input" which leads me to think it's load is probably designed for a guitar and wouldn't be the best load for a preamp. But you could try it and see how it did ..... won't hurt anything.
I'm thinking you can probably plug the preamp into the line/mic input and in the menu somewhere you'll find a panning function.
Let us know.
Hopefully someone who has one will come along and patiently explain that I'm an idiot.