2+ weeks in starter cubes?

High VitaMan, Yes, for 2 weeks is ok. Happy Growing Buddy! 💚

Yes. I would plan a strategy ahead of time to deal with the excess roots from the cube. Taking reasonable steps up in PPFD is preferable I think. I don’t know the distance your 45watt light will be vs the 300watt. But maybe you already have this process planned out?

I've kept rooted clone cuttings in plugs placed in just net pots with hydroton for several weeks(like 5+) just irrigating them twice a day before transferring them to a reservoir. It will work for seedlings too. It helps if you keep your plugs in some type of tray that helps keep your roots covered and prevents them from drying out as fast. Just don't let the roots dry out....but don't soak them either lol....really just misting the roots a couple times a day is all they need. Honestly I would worry more about stretching keeping them that long under a 45w. I'd run it a little extra close.

Not ideal. You really want them out of something so small. It'll start small. Small main stem, as it can't build root arms and expand. And that's what forms bigger main stems. In turn gives stronger larger plsnt base. A better Base tends to give better finish or makes it easier imo Prep is crucial I think. And 2 weeks of week stems vs 2 weeks of even starter 2l pots. Well I know which I'd pick. Also. If you're using hydro. Or soil. You want a bigger area for roots and root arms to branch out.

Its not ideal, but it can be done. If this is DWC then it should be a bit more ok. Let the roots explore and transplant carefully. The bigger issue is 45w to 300w and it will burn the leaves I suggest you dim that 300W down a lot and slowly work its way up for the leaves to adapt or they may burn.