You are probably better going direct to the PA with your SM57, but do not use an XLR to jack, use an XLR to XLR to minimise mains-borne hum. Of course if you need to hear yourself you will then need to be mixed into the monitor and at that point you are dependent on the soundman to achieve audibility without feedback.

I reckon you are better with an undersaddle. Brian Rodgers built me a new bridge for my (acoustic) Kentucky Flatiron mandolin with a B-band undersaddle and endpin mounted preamp - it's a VERY tight fit, but it is very hot and fairly true. Use the 80 hz shelf on the mixer to reduce thump.