After a few days of slow correspondence (the Support system is based on a one question-one response model, you can't respond to a response without it going to the bottom of the Support que) I finally received some answers regarding some questions.
Nothing affects the drop rates of Inferno items other than playing in Inferno difficulty. The direct and slighly more ominous quote was "As for your drop rate question, nothing changes the rate." So I imagine sticking to the boss at Inferno 1-3 is sufficient for Tier 16 farming, unless you want to build up more EXP as well.
Additionally, several days before their implementing the Lock/Unlock feature I had accidentally sold one of the two Inferno items I had managed to acquire through the store; I wasn't wearing it because I had found a Tier 15 Rare with ~130% as much of a final Attack rating, and forget to uncheck it when I was sleepily clearing my inventory of the stuff I'd mindlessly farmed while watching a movie.
First, they asked for confirmation of any monetary payments I'd made in-game. Then they responded with a code for 100 souls. Which I guess they seem to consider an equivalent value to an Inferno item.
Really, what it feels like is being compensated for a lost jackpot with a free pull at the slot machine, because the only other option is to believe that there's no in-house understanding of the monetary equivalent value of Legendary and Unique items given the low drop rate in the store and even lower drop rate from enemies.