I tried using some similar app with HTC Desire X, it would become hot and eventually powercycle.

I remove the back case on my Samsung s2 and it seems to help drastically

As opposed to only the microphone and location tracking being on 24/7?

Yes, I guess?

I haven't used a camera on a device 24/7, but I have used multiple tablets as a camera monitor for months on end.

What I've found is, as long as it's reasonably modern supporting fast charging, the device has no problem staying on with full screen brightness.

Some older devices can drain their batteries quicker than their chargers work however

I've had Waze burned into my Samsung Galaxy S7 screen when I was commuting 2-4 hours a day for a 3 month gig...

That is because a Samsung uses OLED which suffers burn in (since blacks emit no light on these screens). Newer phones do subtle shifting of long running pixels to avoid burn in. You can “fix it” by putting a light gray image as full screen for an hour or two a night. This will eventually ruin your phone... my Samsung s2 and note 4 which I use as a on-the-go baby monitor are almost useless for reading any kind of text