Your thermometers should be at the snakes level, not at the top of the tank. Also, they should be digital. Those round gauges are useless.

To be perfectly honest, there is nothing wrong with leaving a prey item in with a snake unattended for short periods of time. Less then 30 minutes will be fine.

Serious feeding injuries happen when a feeder has been left alone with a snake for LONG periods of time, overnight or for days. Almost all the breeders who feed live just drop the feeder in and check on them again in 30 minutes. If it hasn't eaten then the feeder comes out.