Filming inside is only really needed if you do uber or other form of ride share duties.
If you live in a place where cars only have 1 plate, you will be better off with a front + rear camera, as that rear camera will be your only chance of capturing a plate on a on coming car with just 1 plate in the back.
HD,,, Yes you want 1080p at least, there are also 1440p cameras now and of course 4K ( also some fake 4K which extrapolate 4K from data from a smaller resolution sensor )
Today most cameras work well at night time / low light, but in those conditions capture of little details such as license plates are often right out the window, or take a very specific set of circumstances, where very low speed are the prime ingredient.
The sensors in play today are ( 1080p ) Sony IMX 291 - IMX 307 -IMX 427, for the 1440p cameras the IMX 335 seem to be the most popular sensor, and for the 4K cameras the IMX 415 seem very popular.
Be careful not to focus too much on plate capture, it can easy be done, but just as easy you can miss one too, take comfort in what the camera do 100 % day or night is logging your own driving in relation to roadside / lane markings / intersection colors ASO, which in most cases should be fine fending off the most claims against you.
But true the hit and run scenario, there you would want a plate capture, but it is not guaranteed at all, if you can you are best off seeing the plate yourself call it out for the microphone to record.
It is a good idea to turn off a car in case of a event, at least a major one, though than it is more a thing to do for people coming to the rescue, i think many new cars also do this automatic these days.
But you are right in keeping power on in case of a minor event as things can get hazardous in the aftermath, and if you can get the other part lying to you or maybe police, you will be able i assume to nail them on insurance fraud too / lying to a police officer.
So always let people dig their own grave if possible, and dont tell about your dashcams, at least here in Denmark you are not obliged to share anything with anyone at the site of a event, here i would just hand over footage to my insurance, and then it is their job to detect and prosecute any insurance fraud ASO, where of course you might be asked to give evidence supported by your video.
That is "also" why you pay what you do for your insurance.
Dashcams can go into parking guard mode with the key off, but here the options are often motion detect - time lapse ( no sound ) or low bitrate regular footage ( which i prefer )
The parking guard can be powered by your car battery or a dedicated dashcam power cell which is a extra cost.
no matter what solution you use here, you will need the proper hard wire kit for the camera you have, this will have a low voltage cut off so in parking guard your camera can not totally deplete your car battery, it is not recommended to use below 12.2 volts as cut off ( 50% depleted car battery )
Also you of course have to drive / recharge what you have lost of power during parking guard, so you can not use parking guard for 23.5 hours a day and just drive 30 minutes daily, that will not add up even if a car battery charge much faster than a dashcam discharge the battery.
Personally i use parking guard for just 1 hour on a timer in the cameras i have, this cover all my shopping and at home i have CCTV from my 2 floor apartment on my car parked in the back yard.
This is better as the CCTV have IR light to aid in total darkness, parking guard at night in the back yard would be useless to me as it is simply too dark for the camera to see anything.
When you read about dashcam power sources internal, these are very little ones and they are only there to finalize the last recording in case of a major event where power is cut abruptly, and as i said above some new cars will cut battery power at the battery or very close to it, and in that case i think only EMG flashing light will work.
In case of a minor event i would just turn off my motor but have the key in ACC so the cameras would still record ( my cheap bottom of the barrel Suzuki car do for sure not have any automated power cut off )
you should be looking at dashcams with capacitors for internal power source, the battery ones tend to have the battery die sooner or later, capacitors stand a better chance of surviving low / high temperatures which lipo batteries dont really like much.