I was just in line to post a blurb about why heaters may fail so some of the info is relevant to your question about the "bimetal".
While there are various ways to control heaters, this is kind of prepresentive of many brands. This is a really cheap Penn-Plax while others use somewhat better parts but still many use the design and they have the same problems.
This is the layout of the main parts.
Rather than step all over your build I will post the details of heaters in a seperate item.
Your build looks like a nice job that should avoid many of the problems with heater failures.