When to start veg?

there's no rock solid line to draw here. As far as i'm concerned and for what i want to track and adjust to maximize, when it can handle full DLi and full diet, it's more than a seedling. Again, these distinctions are not black and white and fairly arbitrary overall. i count vege from the day it sprouts. I use this sort of collected data to understand what is expected... and to justify any adjustments made to the process. Because i had a solid baseline of expectations i could easily see the value of a heat mat (colder climate here) and adding moisture (dry in this season). I could see how it improve consistency and sped up early growth relative to past outcomes. I could see tht adding myco didn't do shit for any of that, lol, so i quit buying it. However you choose to cont days or this stage of life or that... just be consistent. Don't choose to do things in a way that adds more fog of war if trying to compare past results.. keep it simple. if it's a soilless context, the typical target amount of nutrients around roots is safe for seedlings anyway... ramping up light is about the plant in front of you and not some pre-ordained time frame that doesn't necessarily relate to every single plant you grow. simply react to the plant... what 'should be' does help, but never ignore the reality in front of you.

I would count them separately. Also full leaves may not start until like a month in. Do it however you want. I count day 1 veg from the day they sprout. I still consider them seedlings but I still consider it veg. Doesn't have to be 1 or the other ya know

It's totally up to you, what I do is give a week for propagation and by then there's a set of leaves out and showing the second set this is where i go to 1st week veg..