Q1) My Dinamed #2 is looking a little concerning. ...

Sometimes called “yellow leaf spot” or “leaf septoria,” this condition is caused by a fungus (or sometimes a bacteria) that attacks cannabis plants and usually appears in warm, wet weather. The symptoms first appear on the bottom leaves of the plant. Immediately (and carefully) remove all affected leaves to get rid of as many spores as possible. Prune any leafy parts of the plant to improve circulation through the plant Avoid getting water on the leaves or laying on top of each other – you want to avoid moisture Make sure to keep the ground under your plant clean. Rake away all leaves and vegetation. Adding mulch can also help prevent spores from spreading. Keep plants healthy, sicks plants are much more susceptible to leaf septoria than healthy plants One way to help prevent this fungus from attacking your plants is to rotate crop sites or move to a new location every year. A copper based fungicide or a broad spectrum fungicide may be effective at stopping the fungus from spreading though they are best used as a preventative.

It's not magnesium deficiency, it is leaf miners. As for the yellowing tips, it is nute burn, Your plant is too young for nutes; does not matter how old they are, what is important is how well they have developed. Stop all feeding and spray something on the leaves before lights out for the infestation: neem oil will do, would be better with pottasic soap.