Kiva Fireplace Firebox Expansion Question

I am currently designing a kiva fireplace that will be built into an adobe home, and have run into a possible issue that I can't seem to find any online guidance on. The issue concerns the expansion of the firebox (clay firebrick) and how that may affect the curved, plastered, front face of the fireplace. Because there is very little documentation online for kiva FP construction, I've had to try and adapt standard FP code and directions intto my design. I've seen a lot of advice for leaving airspace between the backup wall and the firebox to allow for expansion. I've incorporated this into my design, but because the front face is tied into, and bonded with the firebox - as well as the backup wall (refer to attached pics), will thermal expansion of the firebox cause enough movement to affect the front of the FP - i.e. cracks in the plaster, mortar joint failure? I saw where someone suggested just filling the cavity between the firebox and backup wall completely with mortar - making a solid, composite wall, and then incorporate the airspace between that and the adjacent masonry - which in this case will be the adobe brick. But I'm concerned if I do that, any thermal movement within that whole structure could cause mortar joint failure. Any insight into any of this will be greatly appreciated.

Note about the illustrations - there will be an additional four courses above what is pictured for enclosure of the smoke chamber. The dark red bricks are clay firebrick and the pink is standard brick.

Thanks in advance,
Frank Williams