No, it isn't. And no, there are none.

What's more relevant for normal typing is to look at my Sym mod. As you can clearly see from the Programmer Colemak site, the underscore-hyphen and quote-doublequote keys are used heavily in coding. But they're also used much in normal typing!

Look at the BigBag ergo page for more info, under the Sym mod.

( Θώθ)  ～(  See "DreymaR's Big Bag Of Keyboard Tricks"  )
  https://dreymar.colemak.org

How am I supposed to answer that? If you're set to learn it, contact its creator. Or, you know, do what most people do and just learn it. With your mind, you know.

I don't think very many people use that particular AltGr layer. I'd much much rather use my own, of course. And so would most people who made such a layer.