It makes most sense to me to keep ABXY as Home/End/PgDn/PgUp, for backwards compatibility and because it just makes sense to arrange them like that (a cross shape where up = pgup, down = pgdn, left = home, right = end, this is intuitive).
The two extra buttons could be anything, but Ins/Del makes a lot of sense because they are missing on the Pandora and they are useful enough to deserve dedicated buttons (e.g. I use Ins a lot to select files in mc).
Mouse clicks should be done with the nub clicks imo. Remember that the Pyra nubs will also be a button that can be clicked. AFAIK you can click the nub as long as it is close to the center position, so you could click+move (drag) but only at low speed. It makes most sense then to have left nub = mouse, right nub click = left mouse button, right nub = whatever (scroll wheel?), left nub click = right mouse button. That way you can do easy left click dragging.