[dmun]

Wow Wow Wow. Cellulose insulation is wood fiber, basically recycled newsprint. It's not a high temperature insulation. I know it's treated with a fire retardant, but this is way out of it's design specs. This sounds like a bad idea. Here's an experiment. Put a pan full in the home oven. Heat to 400f or so. See what the house smells like.

I built an oven on top of a fireplace, but the openings were 180 degrees apart. You can do it, but it makes your project much more complicated.

Your design sounds way too small, even for what you plan. Remember you need room for your fireplace flue to circumvent your oven, and room for insulation between the fireplace flue and dome. Also, building an two-flue chimney is a non-trivial exercise.

Some of the barrel vault folks here may weigh in on your plans. Please give us some sketch, even if it's a digital photo of your back of the envelope sketch.