Dataflows and the enhanced compute engine

I would really love to see many of these questions answered Anonymous as I'm experiencing similar headaches to you, very frustrating are all the lengthy waiting periods working with PowerQuery in the service for such things like preview results (inparticular with merge steps), endless issues eith refresh timeouts, and even to simply do the validation required to save & exit out of the PowerQuery 'edit' screen can be herendous for me depending on the complexity of what i'm doing.. but even something as simple as changing a few data types can have me waiting literally an hour+ for the query to validate and save.. hair-pulling stuff I'm sure you'll agree). On average I've experienced about 80% if time watching spinning wheels, and 20% actually getting anywhere with progressing a relatively simple dataflow model (with entitites of around the same size as yours 6M - 12M rows (we're on P1 Capacity however)) towards completion.

Besides all that.. all I have to offer you currently is that yes - you can actually view your M query inside the PowerQuery editor in the service - just right-click on your table in the 'Queries' pane and choose 'Advanced Editor'.

Best of luck getting some helpful 'real-world experience' responses to these questions!