Yes. When a permanent is copied the copy is exactly the copied permanent with any changes made by the copy effect. The ability in question does not change the types of the copied permanent, just the P/T, so enchantment Volrath will once again be a creature.

So, I misread the question until literally just now after I posted that. Yes, the ability will trigger. It will not be able to go on the stack until the Scion activation is done resolving. but it will still trigger and will be put on the stack the next time a player would receive priority.

Scion puts the card into the graveyard and becomes a copy of it as the resolution of a single ability. No player receives priority and triggers will not go on the stack (much less resolve) before Scion is a copy of what it searches out. The Colossal Grave-Reaver trigger will still see the card entering the graveyard and trigger, eventually putting the card into the battlefield. Scion will just be a copy of that dragon before it enters.

Relic of Progenitus doesn't draw 4 cards in a turn. Ketramose says "Whenever one or more cards are put into exile from graveyards..." since it is a single effect exiling from multiple graveyards it is 1 Ketramose trigger. If it said "...from a graveyard.." it would count each graveyard individually.

Still a great card for a Ketramose deck since it is 1 mana artifact that taps to draw a card with Ketramose on the battlefield. It is a draw engine though, not burst draw. I think that making some cuts to add in Relic of Progenitus and similar repeatable exiles as engines with Ketramose would go a long way to feeling like your deck is "doing the thing".

Depending on budget good repeatable draw engines with the commander could be:

Agatha's Soul Cauldron

Crook of Condemnation

Ghost Vacuum

Unlicensed Hearse

Necropotence is a good draw engine as well and would let you get Ketramose online at will. And Triumph of Saint Katherine seems like a good value piece.

Sure, if you can somehow remove either of the pieces in the loop it will break the loop. That wasn't the question though. The question involved exactly two cards. There are any number of third cards that can break the loop.

The second paragraph of your question perfectly explains the interaction. You control both triggered abilities and can order them in whatever order you would like. You can retain the monarchy, or pass the monarchy depending on what you want to do.