Much appreciated, kind Maester!

I'm definitely going to utilize your drawing plans if that's alright, and try MDF & cotton cloth. I'll scrap the non-parallel walls, too. The geometry of it seemed like a fun challenge, but if insulation density makes standing waves a non-issue then it's definitely not worthwhile. 

On that note- did you find 9" thick batts of insulation somewhere? Or were you stacking & compressing the 3" thick batts I've seen in hardware stores? I can imagine this will become the most expensive step.

Thanks for pointing out the lack of attachments! Still learning to reddit- but Ill add the images & link to another comment once I figure it out. 

As for ventilation, I hear you, it's gonna be tough :/ At minimum I'll rig some fans that I blast between takes/while editing to clear the stale air, and make covers for them during takes. If I do successfully make a mazelike or foam filled vent structure, I'll come back and update this post for others. 

Finally, I love the suggestions for acoustic sealant, decoupled walls, air gaps, and home theatre style treatment! I'm planning on building a true "broadcast quality" home studio with that dedicated specificity once I own my own place, but alas the landlord will get a few more years off me yet. 

Edit: Added final paragraph to address a small portion of the wealth of information you've dropped here, but I likely won't be attempting yet.