Hi there,
So what you list is well within the capabilities of the xiao, The OV5600 is a much better camera than the stock one and has more of the features one would need for a project like your describing. Take them one at a time.
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YES, either will work and I believe I may have a demo video of it working on here.
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The power consumption is High with stock CAMERA so Prep a Good Battery. It does sleep well but not the lightest sleeper of the ESP series power wise. The problem you will face and as I see it is:
"Therefore, the testing was conducted using ESP-IDF, actively disabling the functionalities relevant to the sleep modes to complete the testing. This forms the basis of our testing.
There isn’t any code I have seen to do this???
This REALLY should be looked at Seriously and if there isn’t a Vendor Solution Make a change to the board to fix it. IMO Such an impact on making the thing more than a " GIMMICK device"
The wiki and GIT on this is very good too!
- Grove - MOSFET allows you to power devices or projects that require a low voltage (e.g. 5V) with a microcontroller, such as a high voltage (e.g. 15V).
Post up any additional concerns or questions. , I like the concept, but wouldn’t a wireless temp sensor attached to the item be better. They do make specifically a “Flame Sensor” also just FYI. 
HTH
GL 
PJ 