Try to aim for 90 degrees warm side, 80 cool side. Not need to drop temps at night. That lamp dimmer/rheostat should help you there.

The loose stool maybe just from it eating mouse pinks and not really getting enough of what it needs. It could be an indication of poor health but probably at this point I'd just keep an eye on that. If it continues to have loose stools then a fecal sample taken in for a fecal float is likely your best bet.

You'll notice the snake passing three things from it's vent....

1) liquid urine - typical pee
2) solid waste in the form of feces - pretty much looks like small cat turds
3) urates - white or sometimes yellowish white, they come out soft but quickly harden up, just another form and process of it's urinary function

Putting this snake on a better diet and off those mouse pinkies will likely help a lot but just keep an eye on what goes in and what comes out (pretty much like a human baby that way except they don't poop near as much )