I am most definately not a good authority to discuss what is/isn't menhera, but I feel the need to have a place to answer some questions due to the nature of the topic. I will link much better people to turn to.

What is Menhera?

FYMenhera did a much better job explaining than I ever could. They are the OG for information relating to menhera.

Can I get involved if I'm not....

Never believe you're "not sick enough" or that your struggles don't count.

Isn't this glorifying/promoting mental illness and self harm?

Absolutely the fuck not. The intent is self-expression, not hiding your struggles, and getting help.
Menheratic explains again in their FAQ and I would like to share a good take by Hikidreams

Why no -kei?

Kei (系), contrary to popular belief in the west, doesn't mean "fashion" but "stereotype". Adding -kei makes it 100% derogitory and googling it gives back a lot of awful, very sexist, articles. Think "crazy ex-girlfriend".

As such, no -kei. Ever.