"Also, can I at this early stage give them more darkness, but not 12H, to try and lower the growth, maybe even lift the light up some way."

I think this will make them grow taller. I turn off my lights for a couple of hours a day during veg so they can stretch a little. Putting the light further away will also cause them to stretch to reach the light. When the light is close, they don't have to go up. They grow out to the sides. Not sure how often your soil needs watering, but try to keep the temps down around the low 70's so they can go longer without needing water. The hotter the room, the quicker they'll dry out and need more water. Also, if you can keep some standing water in your grow room it will help raise humidity and get you by until you return. I'm also gonna state the obvious by telling you to water right before you leave. They won't be in optimum conditions, but I think you plants will be ok when you return. Give them a nice dose of fertilizer when you get back and they will be happy you did. Hope this helps.