Hey Skippy
Great to see all is going well - the plants are looking good.
You should try and get your ventilation properly sorted now rather than leaving it too late and if you don't already have a thermometer, I'd strongly advise buying one as they're really cheap and that way you can see when temps are getting dangerous. An extended period of excessive heat could kill your plants or set them back a lot, particularly if they're not getting much fresh air. If you only have a small space, the more they grow and bush out, the warmer it will become at the plant's level and you might find in a week or two they start getting a bit smellier which will make it harder to just open the door for ventilation. In the meantime, you could try opening doors a bit wider and hanging a large black towel over the gap. This will reduce the light leaking out a lot but will still allow air to flow. If you combine this with a fan, especially if you position it at the level of the light, pointing outwards so it's sucking the hot air out and blowing it through the towel, this might help for a while. If you just put a fan in there and close the door, the longer it stays closed, the less effective the fan will be as eventually it will just be circulating the heated air. If you have a lot of vertical space, making a hole at the top of grow area and tilting the the fan to blow upwards might be more effective than just having it blow on the plants itself. I'm not sure what kind of light cycle you're on, but you might find that having lights on at the coolest part of day (usually around the early AM) and off at the hottest might also help a bit. Even having a window open in the room your grow area is in might help to keep things a bit cooler.
Because your plants are still quite immature, if you flower them now you'll probably have to wait a bit longer to determine sex than if you'd let them mature a bit. It would probably pay to let them grow at least one more proper node before flowering.
Within the next couple of weeks you'll likely notice more outward growth as well as upward and the biggest leaves tend to be on the outside which could be a problem if you don't have too much horizontal space. The width will almost definitely exceed the soda bottle within the next couple of weeks. If space really becomes an issue, it might pay when starting flowering, if you can afford to keep your lights on 24hrs, to flower them in cycles so that while 2 plants are in the dark for 12 hrs the other is in the light and switch them over every 12 hrs and that way they wouldn't be so pressed for space. The dark area could just be a big cardboard box or cupboard etc, as with no lights, heat will not be such an issue. This will be kind of a hassle as you'd need to make sure you were always there twice a day for the changeover but if you could manage this for a couple of weeks, at least you'd be able to sex them.
The stem will thicken and strengthen a lot as time goes on but if you did want to bury it when repotting this would be fine as long as you don't go deeper than the first set of leaves.