Urn of Sacred Ashes *spoilers*

Wynne screams at you that she never expected you would go that far, and attacks you. So does that spirit guardian. I was with Wynne, Shale and Morrigan. Shale and Morrigan didn't mind, and helped me take down Wynne and the guardian.
Then Kolgrim gives you the reaver specialty.

Actually I played through the scenario first, not putting the blood in ashes, then I fought Kolgrim, left the area and saved.

I really wanted the Reaver specialty, so I replayed the whole thing to get the Reaver specialty unlocked, and once unlocked, reloaded my save where I still had sweet, unsuspecting Wynne with me, but if the specialty is unlocked, it's unlocked

I just hope that siding with the cultists isn't the only way to unlock the Reaver class.

Actually I believe it is. There aren't any books or other trainers available for the reaver class, AFAIK.

To avoid any party members leaving, clear up to the final room, then go back to camp. Replace all "good" members (Leliana, Wynne, possibly Alistair) with "evil" members (Sten, Morrigan, etc). Then go pour blood in the urn. When you go back to camp, you'll have to pass certain persuade checks, but it shouldn't be much of a problem if you have invested a few points in persuade.

(in both scenarios you have some bosses to fight)

The difference is that I completed the game as an ally of the crazy cultists. Also, the dragon has a ton of loot - the guardian has none. That's one of the reasons why I will probably not choose that path more often than I have to.