Your questions are questions we should ask each time we germinate a new to us plant. Most if not all issues you ask about vairy from plant to plant. Research each plant you are raising. I presently have tomato seedlings that stay outdoors around the clock when night temperature doesn't get below 45f. On days plants are indoors they are in a South window with foil on right and left sides to reflect early morning and late afternoon sun then lights come on 1 hour before dusk and go off 4 hours later. On overcast days when indoors,lights are on from 6am to 10am and again 4 hours in evening to supliment weak sunlight.

Plants don't have clocks. They don't know if it is day or night outside. In general, the more light the better, though some say that six hours of darkness is helpful. With higher temps, plants will generally grow faster. Probably won't make much of a difference whether they are warmer when the light is on, though I guess that's a natural way it happens.

I do 16 hours on, 8 hours off once the seedlings germinate. This is where a multi-shelf set-up comes in handy -- the germinating shelf has 24 hr light (generates heat), the other shelves are 16/8. Plants need some amount of darkness for regenerative purposes -- it's not the time of day that matters under the lights, it's the darkness. If you're growing on a windowsill or near a source of daylight, that dictates your darkness schedule.

In general, some seed that likes light for sprouting up like carrot and lettuce and tends to be surface sown or very lightly covered can appreaciate the light being on from sowing forward. Most seed that likes being more buried does not really need light till it starts popping out of the soil. But then you want to make sure it is getting light from that point on.

Plants have circadian clocks- different species run on differing times of light and dark. When they are small seedlings it is usually beneficial for there to be longer times of light or no darkness. As they get older and start their own circadian rhythms some gardeners like to start giving hours of night or lights off to accomidate that. As they harden off it can be beneficial to adjust the night hours for plants akin to real night hours like we adjust their light from indoor to outdoor lighting during the day.

In general, most plants are used to it being cooler at night than during the day. Seeds can benefit from additional warmth from a mat to sprout, but after that it is better to get the sproutlings off the heat. A couple few degree difference usually isn't a big deal for seedlings, it's the usual flux in a house. But a large temp drop or a lengthy one overnight can cause harm to many seedlings. Especially during the younger days of growth.

I do my seedlings in a large south facing kitchen window that gets full sun, and I also run lights there. It stays temp stable on the warm side due to a basebord heater there. I use a couple of little four tier mini greenhouse cages for my shelving. I have a 24 hour on light for sproutlings. Another couple lights that get night time off closer to the actual night hours for older seedlings. I turn them off before going to bed, and on when I make coffee in the morning. From there it depends on the plants night and day clocks as well as temprature needs. Some will get pulled from supplemental lighting to finish off indoors in just regular window light or harden off in the south face of the enclosed porch before going out under caps soon. Hardy greens and a few early roots for the most part. Shadier plants get pulled to the less light areas. Plants that I might want to slow down a bit will get light/temp adjusted too. More coddled plants like peppers and tomatoes tend to keep getting the extra light till they start going out to harden off.

Of course, the circadian rhythm (it's not a clock) in plants just makes them expect a 24 hour light cycle. That cycle may or may not be phased with the actual light cycle. That is, if it's dark outside, my plants don't care if I have their lights on.