AFAIK the only way money flows from the Corporation to the Foundation is by the Co paying royalties to the Foundation for the use of the Firefox trademark. Obviously exactly how that number is determined is a little fuzzy, but I don't think it (legally) can be just any number - it has to be justified somewhat. In any case, the Corporation is not short of money, so if the Foundation wanted more money to flow from it to the Foundation, a shortage of money due to CEO pay is not the reason.
(You are definitely right in the sense that Co money could be used to fund more Co projects. Those are not the same initiatives that would be funded by donations to the Foundation though, as money doesn't flow from it to the Co. Think Common Voice, MozFest, lobbying, Privacy Not Included...)
I mean, depending on what you mean by "purposely", I just think there's no good way in general to determine the exact worth of the use of a trademark.
Since the Corporation was founded on August 3, 2005 - this might've been the reason? Before the Corporation existed, the Foundation had to receive the money from the search engine providers directly (and the "tax reserve fund" sounds like creative accounting to hold on to that money, potentially leading to the audit), whereas later, the Corporation could hold on to it and pay taxes over it like a regular corporation does.
I'm with you here, and I'm not saying that the ratio CEO pay:employee pay is a good one. All I'm saying is that the money used to fund the CEO pay could not have been used to fund Foundation projects like Common Voice, as far as I'm aware.