Re: Seiza by ranks?
In our club we just line up in grade order, within the same grade it depends on who sits down first, sometimes it's me, sometimes not.
Interestingly (maybe) at the Aikikai Hombu people do not line up in grade order, just in lines of people, whatever grade. I guess that's possibly because it would be too difficult to determine who's who and who should sit where with all the visitors passing through. The dojo I train at in Kobe when I visit also works the same, you just find a space to sit. My old club in Tokyo does line up in grade order but seniors on the left, not the "usual" right.
It seems that there is no definite rule, just what each dojo prefers. Personally, I like the Hombu/ Kobe system.