But they aren't actually new Pokemon they're just new forms of them.
No they are evolutions it's even in their name. They are a new type of evolution method yes, but they are evolutions none the less. The only true difference you have is one is temporary but the trade off is that for being temporary they were able to be made much stronger.
It's basically the line between a new form and a new evolution, it fits into both and is the way this generation chose to give new evolutions to old pokemon but not deal with all the whinning of how their old pokemon had been tainted.
Eh they might be on the line, but they're closer to forms than new Pokemon, especially since they're not a part of the National Dex
Not really because a 'Form" is an adjustment to the stats/type/etc... The pokemon doesn't become stronger or weaker rather their existing stats are rearranged to change their focus. Megas don't adjust they just add more and make the pokemon stronger more evolved.
Again megas are basically smack dab in the middle they have traits of both evolutions and forms but they don't fully fit into either category.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me