I freakin' hate it when you're right.
I just called New Belgium (since their kegs seem to be popular for conversions), and asked them about this. It turns out that it is becoming more common for their kegs to be rejected for return by liquor stores because stealing them from another customer in order to return it for the deposit is common. Of course, in this case the keg is unlikely to come back to the liquor store that sold it, so apparenlty they won't take it, even if you're not asking for the deposit back.
But they did say that if I had an empty keg the brewery would come pick it up themselves because each keg costs them $140 making it worth their time. I asked about returning it to the distributor, but they didn't seem keen on that. I didn't ask why, perhaps they self distribute?
I'm still gunning for you on that mash depth v. efficiency thing, gonna do a side-by-side this weekend to check it out. But I'll probably be wrong there, too.