Question about ASUS H170M-Plus

One Single Yes, SLI No

Yes, you can put a GTX 1080 on here, but as soon as you want 2, it only works with AMD cards. Since this mainboard does not support SLI

yes, this is possible. it is best to install it on the 1st slot, as only the x16 supports pci express.

I think so, but if you want to use multi-GPU it is only for AMD Crossfire.
But I'm not a professional, so wait for another comment.