Moon's chapter makes it clear she went through training to learn wandless magic and reach Butterfly mode. It was as easy as saying "don't do it, because once you reach Butterfly mode you can open a portal to the realm and corript it"
Considering how much of a coward Moon is, she would have obeyed.
Also, why would your arms getting tainted stop Moon? She had to save the kingdom and that was the only way to do it. Eclipsa doesn't even have to tell you this, killing comes with repercusions and some ugly veins for ending a life seems like a fair prize.
Seriously, just accept Glossaryck is a terrible mentor and can go fuck himself for being so secretive. Eclipsa saved the kingdom with that spell and neither her or Moon knew about the Realm
Fuck, even in the finale Eclipsa is shocked when her corruption dissapears, meaning she never once tried to use the temple.