This installation is a cause of anxiety for you, obviously you want to do the job right and are looking for the right direction on this forum. I think because you are a little stressed about it, you are overlooking the fact that each of the responses are giving you the answers to your dilemma... that even goes for MPM, you just have to know that he's an old timer pro with plenty of humour to go with his advice (even when he's not trying to be funny he makes me chuckle) - these guys are all a serious pool of information that you can't find anywhere, and they are helping you. It may be hard to see that BECAUSE you're stressed about your project.
You are on the right path, eyeglasses with the valve in the middle. You can take some reassurance in the fact that you have the right valve for the right system, and any extra effort that you put into the installation will help you sleep better at night. There should be some instructions or specs that came with the valve that state: you must have "this size" for the supply, etc, sort of thing. If it's too vague, then contact Moen directly and ask for clarification. If everything checks off the list, then go for it! Get 'er in! If you are still really nervous about the performance, then take that extra effort you're already putting in and try to test the valve/heads before you seal up the wall with tile. It will mean extra work, but should give you that final reassurance you're looking for before you tile up the walls...