I'm thinking a setting in the MOTU interface is going to be the culprit for the first (default) input having problems. But now you are working with ASIO4All, RB, AND the MOTU mixer interface. It sounds like whatever the default is, it's being sent back around through a routing schematic (which would be inside MOTU mixer routing most likely).
Just thought I'd toss my first idea out there as far as the new progress is concerned. If all inputs 'work', and whatever the default input is keeps being the problem ... it sounds like a setting somewhere. Have you spent much time in the MOTU mixer applet? Maybe 'monitor mixer' or some other setting is creating a loop for the default input. I know my Delta mixer/routing can be set up in some pretty complicated ways, and I would think MOTU is at least as complex.

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