1.) the actual physical “Center” of the Dou will produce the loudest sound. It’s similar to Taiko drumming. You’ll want to “Aim” for the center etc. in the case of Shamisen the exact center point ends up being the spot we call “Ushirobachi”. So by keeping your hand, wrist & arm relatively relaxed and just letting the Bachi naturally smack down onto that spot on the Dou, you should be able to produce the tradiotional “Loud” Shamisen note. In contrast Mae Bachi is located far from the center which already naturally causes a softer quieter note. The two basic Tsugaru Rhythms are Ushiro Ushiro Mae + Sukui (four beats like Jongara shin bushi etc.) and Urisho Mae + Sukui (three beats like Aiya or Yosare bushi)
2.) Like Ian S. has mentioned the Ushiro/Mae Bachi is stylistically very much a Product of Tsugaru Shamisen. Other Shamisen styles and genres tend to have a different approach. However in recent years I have come across several players (sorry, names are escaping me at the moment) who are actively combining Tsugaru techniques with other Shamisen styles so I imagine the future of Shamisen might yield some new opinions on this subject.
3,) yes as Ian mentions this is simply a pattern created to suggest or imply a rhythmical framework. I mean theoretically you could combine Ushiro and Mae in any sort of polyrhythmic pattern you like so long as the four beats… U U M S (Ushiro , Mae, Sukui ) are maintained in your mind’s eye or in the background etc. we’ve all heard drummer play solos in which they go WAAAAAAAY off beat but finally in the end they hit the down beat BAM! And we all suddenly realize that the beat was in fact never abandoned for even a split second. In the same way you can literally do whatever you want so long a the pulse can still be felt and so long as you end up right back on Ushiro Bachi in the right place when you bring the original pattern back.
4.) It isn’t very common to do the U U M S pattern on Ichi no Ito but keep in mind that music is Organic. If one player pops up in the spotlight doing that pattern on Ichi No Ito all the time then eventually people will start to say “Oh yeah. That’s totally so something we do.”
In other words if something can be done musically and technically then “Not” doing it is really fundamentally just a question of personal taste or personal aesthetic value or stylistic preference.
5.) Ian’s answer I think is spot on!