The built in temp sensor (vcgencmd measure_temp) does not report ambient temperature but the actual chip temperature of the cpu. Depending on cpu activity level and actual ambient temps especially if a case is used, it will not reflect environmental temps well. I would suggest using a ds18b20, especially the version that is packaged in a metal tube with several feet of lead wire attached. It can be positioned to be removed from any heat generated by the RPi.
Presumably, in the event of a power failure, even if you have back-up power for the RPi, your LAN will go down and hence your internet connection. If your internet connection is immune (for example, a DSL line) and you are able to back up the entire chain...modem, router and RPi, you could get out a message, email or whatever.
You would need some non-connected way to sense the power failure if the network chain is kept alive...if the back-up power provides some kind of a AC failure signal, that would be easy but something as simple as a led that could be "seen" by the RPi and a light dependent resistor would work.
I don't think you (or I) burn any bridges by using the RPi alongside Mint. Except for Mintboxes that are 10X more costlly than the RPi, running an x86 is much more power thirsty. Still, for some things, only a full up x86 linux box will do.
But my Mint desktops don't have GPIO pins or the kind of real world capability of the RPi but when you need a big memory, the 4G of my desktops is going to win every time.