Seedleranswered grow question a day ago I usually top really early (in my grow right now on day 2 of Veg) and i think the earlier the better and easier. If i wait for a longer amount of time, and then top, i have sometimes problems with training because it will split the stem in half, but that never happened when i topped really, really early. Idk if just random or the plant grows better together when topped early, i don't know
BUT i know your plants are angry! Magnesium chlorosis, purple stems. Too much/intense light and not enough magnesium for the plant to protect itself against it (thats why the stems turn purple, its like sun burn)
You need to fix the nutrient problems first before trying to do a cutting or topping imo
So long talk, just do it, why not? what you have to lose? Nothing really, good luck on it!