Yellow marks on some old bottom leaves

it's only a Mg issue if it was low on MG from the start. It takes 5 weeks from the inception of a Mg deficiency to see any symptoms on leaves. your plant is only 5 weeks old.. it is possible, but the time frame is a bit odd. this is only on the 3-fingered older leaves? if so i wouldn't react to it just yet. if it remains only on the 3-fingered leaves i would ignore it. those first couple sets of real leaves, not cotys, often have blemishes. they often get less maintenance from the plant over time -- first to shed etc. burnt tips can be a number of things.. keep an eye on that, but i'd wait for more clear symptoms before taking a course of action. always easier to add things than take away things.