Looks like poor watering habits as well as too much light (mostly the plant at the bottom of the 4th picture in week 2). that might be causing teh n-deficiency symptoms on a few plants. i'd fix your watering habits before making too many other adjustments, as that might be the cause of it all.
that medium needs drainage/aeration amendments. Even if it is coco coir based "mix" it needs 1/3rd perlite. If it is a soil, you want 50%. Doesn't have to be perlite exacty. There are options.