That is a nice setup you have going there. You will need a different light (hps or at least warm spectrum flouros) when it comes flower times.
The only thing I can say about bagseed, it is likely to be sativa or sativa mix, which will want to get BIG, which can be hard to do in a box.
The idea of sexing and cloning is excellent, it will make the overall grow a bit easier as far as size of plant, plus you can get an idea of how big the plant gets to get ideas on how soon to top.
1. How to tell when a plant is mature enough to switch to flowering?
I believe the rule of thumb here is to look for alternating nodes. At first, the plant will be kind of symetrical in growth, but will soon alternate nodes on the stalk (as far as hieght). At this time, the plant should begin to pre-flower, which will show its sex and at which is ready to go into 12/12.
2. When approx. can I sex the plant?
Once you the plant is sexually mature, switching to 12/12 will show sex is abouts 2 weeks. Or if you continue veg, once sexually mature, a plant will begin to pre-flower which will also show the sex in abouts 2 weeks.
3. The height and nature of the cut needed for topping effectively.
This depends on the plant and/or your grow space. Rule of thumb, A plant will abouts double in size once you flower during the stretch phase. If you have 12" of grow space, you need an uber short plant. Really, clones are almost a must here. You don't want your plant to be more then 6 - 8 " tall and even then you will be doing some tieing down. I would say topping abouts 4 or 5".
You can keep that plant rediculously small if you woork for it. To make a Bonsai Mom plant, I top a clone once it has 4 nodes, maybe 4" tall.
I would heavily suggest some research on your part before making any hasty moves.
There should be videos on growing and texts about topping and FIMing (a form of topping).
