I use and teach index finger tip at the corner of the mouth. I like the alignment of bow arm and draw arm this lower-than-the-eye anchor promotes.
When you get tired it would be best to take a break rather than let your form deteriorate -- I know it is hard to quit because shooting is so fun.
Shooting with both eyes open is ideal if your are matching the dominant eye with the drawing hand: better depth perception, peripheral vision, and for some, better balance.