Well, I suspect no one's replying because we know we have differing opinions on this. It's all in how you translate the Mazda 5 owner's manual. It does state to push the 2nd-row seats all the way back when installing carseats, but it's debatable whether they state that as a recommendation for ease-of-use, or as a firm rule that has to do with a safety issue.
I'm not sure what the newer manuals say on this, but I personally (saying this with my tech-hat OFF) choose to interpret it as a suggestion for ease-of-use. Technically, the manual says to push the seat back, so as techs we can't do much more than just shrug and say "That's what the manual says." As with any decision, it's up to you to decide what to do, yk?
Also, just to make a note about the seats - you can fit two Husky/Regents side-by-side in the 3rd row, so width is definitely not going to be an issue with any carseat/booster combination.