I tentatively reply to this post as I'm not actually what the op is asking. However I see simalarities with a number of design industries where big (or 'bigged up') names get involved - I have no idea who your designer is but I assume he fits into one of these categories.
Correct, he was a "big wig" at Dolby Labs...
What I witness is that when a large budget is available then the designer starts to 'experiment' at the clients expense which basically means that the designer is no longer the expert they once were so things like budgets rapidly get out of hand or become difficult to estimate accurately.
I do not know if this is what has happened in your case but if may be worth asking the designer to base his design on more convential principles where things are tried and tested and costs are easier to accurately predict.
I do not think that this has happened as everything that is asked is explained in layman terms and, to me, seems reasonable...again and as you note, one of the problems is that I am not an acoustic engineer and therefore can not be sure but can only go by my gut which tells me that he and his company are nothing but honest and employ time tested and prooven approaches...
It may be that your designer is a primadonna and has a hissie fit or they may just modify their design to bring costs back under control.
Quite the opposite as they are working diligently with me in addressing the budgetary concerns...whether they get there or not remains to be seen...