I’ve thought about a few of your problems, and here’s what I came up with:
About the strut tops:
There is a ridiculous amount of parts that are interchangeable between the Cuore, Sirion and YRV. I know there are adjustable strut tops available for Cuores, in both camber and caster. Eventhough they’re mostly from Malaysia, they do exist, so they may be available closer to home as well. I wouldn’t be surprised if you can use those for a Sirion. I happen to have a YRV donor car atm, so I could compare those struts to the stock ones from an L7 (I’m driving on BC coilovers myself).
About the rear axle:
Now this may seem a bit extreme, since you’ve put a tonne of work into your AWD swap so far, but maybe it’s worth thinking about FWD. Obviously, AWD gives you more traction, and it makes sense why you did the conversion. However, I’m wondering how much you actually gain from the AWD system. You know the car best, so maybe this doesn’t apply to your car, but these are my thoughts:
FWD means a torsion beam rear axle (in case of the Sirion). The obvious advantages compared to AWD, are less drivetrain losses, less weight, and a simpler construction. But that may not be enough to make up for the traction disadvantage. I think the biggest difference will be the handling. When cornering, the outer wheel will gain camber and toe out, which will result in the car wanting oversteer more (which might help with those tight corners). Relatively though, because it all depends on the rest of the car. On top of that, you can put a swaybar on the rear axle. I’m not sure if all Sirions got one from the factory, but I know they exist. Anyway, that’s another option for adjustability.
What I’m trying to say is, that although you would lose traction (especially from the start), you might be able to gain some time because of the decreased weight, decreased drivetrain losses, and improved handling. I’m guessing you’ve already thought about this, but it might help.
If you want to go independent, I guess you’d have to get the rear suspension from a Charade G200 or Gran Move, as they have independent rear suspension. I don’t think it’s worth the hassle though.
Edit: I might have used the term ‘torsion beam’ incorrectly. Should have been ‘twist beam’.