Using old Android phones as multi channel dash cam
Most of your requirements can be met by either a custom software or some apps on the Play Store.

May be not merging video. You will need to upload chunks of video periodically, be it hourly, daily or weekly, to some central location that knows about all devices. (Some server in the cloud? your home PC? May be you can set the save location of the apps to a OneDrive/Google Drive folder)

I don't think there's any harm to the camera on devices on atleast most devices. I've seen people use droids for home surveillance for a while.

Although you'll have to make sure the device isn't placed in direct sunlight and not exposed to extreme summer heat. Most devices can shut down automatically when the heat is more than it can handle, though it is mostly to prevent heat damage from an overloaded/misbehaving CPU, not the heat from the environment.

Battery Life

Main issue would be the battery life. Even when simply recording by using a charger from the car (usually 1A. May be 2.4A if you're really lucky to get one of those charger adapters.)

Your camera won't last more than a hour or two under it's own power. (Even the latest devices can only last 5-6 hours with screen on) You can may be attach a battery pack which uses solar panels to charge.

I don't know the effect on the battery when its connected to a constant power supply. Many new units can handle a good amount of abuse, but older ones may have issues. Although this isn't an issue for you as long as you have a way to constantly power the device.

Things may become a bit simpler if you only need to record when the car is running. This will reduce the battery issues significantly.

Due to the openness of the question, you may have better luck posting this on a discussion group/forum or something like Reddit.