Im not sure I get your last discussion point. I don't think the 'swing arm' discussed in the diagram is a physical link. More of a virtual equivalent to a control arm to make visualisation of the roll axis easier.
I'm sure with more links (upper for example) you would change to roll centre height. The axle would now rotate around different pivot points (or worst case bind if they do not work together). I would think that an upper link of the same length as the 'swing arm' would be a good starting point.
Overall roll centre height adjustment would be better managed with a watts linkage and slippery link between the axle and leaf spring.