@SpannerI am not a GM but I can attempt to answer to the best of my knowledge. I may generalize the first because that is more determined by the GMs.
1. From my personal experience in this RP, if your magic is extremely destructive, largely unstoppable, or unavoidable then there is likely a problem. The other issue comes with being over versatile. If you can do everything then some or all of those magics should suffer in some way.
Take my character for instance. Ariel can essentially copy magic types. Her potential for being OP is through the roof given that she could potentially do everything. However I knew that so I put drawbacks. First she has to encounter them enough to copy it. Then she starts low for each of those magic types and has to actually use them to grow them. As the breadth of magic available to her grows her time has to be divided among them. So some of those magics will be stronger than others because they're used more, while the others lag behind. She also doesn't inherently learn advanced spells and has to either use more generic spells or learn the stronger ones. Copied slayer magic is a little special but not common or utilized often on purpose. Did I perhaps make it over complicated in an attempt to make her less OP? Probably, but it worked and I think it makes for a more interesting character.
2. Most of the slayer magics are scrutinized pretty heavily and generally not accepted. The one exception at this point might be Demon Slayer because most of those characters are no longer active. However slayer magic is usually discouraged because there are enough as is and they are supposed to be rare. Lost magic and black arts are also rare but there aren't as many so you're probably safer with that. And yes they all have to fall under the power restrictions.
3. We are currently at the beginning of a new arc. I'm not even entirely sure what is coming next. The previous arc was thefirst Grand Magic Games in over 100 years.