Hi Simon,
Although I'm no rigging specialist I might take a stab at this and I'm sure others will have better ideas, but looking at your screen shots I think you are asking rather a lot from not very much model geometry!
(I've scribbled some first impressions)
Whilst keeping the patch count low is generally a good thing, for areas like joints more can sometimes be better!
Whilst a lot can be achieved with good CP weighting some Smartskin will also be needed to pick up the slack as it were.
Its also with considering with things like hands, feet & heads do they need to be all modeled in one continues geometry? A separate hand pocking out of a shirt sleeve is free to rotate without twisting the arm mesh at all.
If I may, I might link this to your other post on project management.
When planning a character always ask yourself what am I going to want it to do? and can it do it? This doesn't just apply to complex movements but also to the seemingly simple ones too. Extending this further, conversely don't spend time in making a character capable of things its never going to need to do.