-Does this look like the beginning of a magnesium ...

It doesn't look like a deficiency to me to be honest and you are already giving her calmag,no problem.more like a leaf tissue thing,no big deal. I would go by 4th node.at this pace,probably at the beginning of week 4 Your plants have tight node distance which is a good behavior in my opinion.only could be challenging when trying LST on it. 100 cm light distance is a lot but your plants are not stretching to the light which comes from their characteristics.they would have stretched a lot if they wanted.bring the light closer for better penetration and they'll grow a lot quicker and stronger.no need to search for extra height at this stage,all going cool at the moment.