RHat, I also took a look at Street Magic when this question was first raised. And nothing in possession relates to that.
1. How was it in SR4? Since the part about not having any weight is more or less fluff text and not mentioned in any way for possession, why do you think a possessed being would somehow become weightless? I would approach it the other way: It's not stated that they keep this ability

2. Of course they might have forgotten it. But who knows? They don't get it by RAW.
3. Are drugs supposed to work differently for paracritters? Drugs are toxins with both a positive and a negative effect. Since they don't make exceptions on what they can affect, you could feed some Kamikaze to a possessed being

4. Vessel+Spirit count as single dual-natured entity. The former beings don't cease to exist, they are merged. I don't think that physical spells would be ended, as they are cast at a physical representation, which does not change. I could understand that manaspells fizzle, because the targeted aura has changed. But I have no reference in the rules.
For the case of the vessel itself sustaining spells, I think they would end.
5. I didn't find anything related neither in the possession power nor in the sidebar of Street Magic. Your source?
6. Shapechange does not replace your body. It transforms it. Cyberware is not excluded from shapechange, so it would be unaccessible while shapechanged.
7. The modifiers are random. Fire should have +4 instead of +3, Man should have +3 instead of +2 and it gets even worse because Guardian has +1 instead of +3, Guidance has +0 instead of +2.
For Fire and Man it may be because their higher Intuition is not factored in, but both Guardian and Guidance spirits receive a higher bonus to their REAction than their Initiative bonus indicates.
That's why:
You calculate Initiative as given with Fx2 +modifier +2d6. An Increased Reaction from the possession thereby has no Influence on Initiative.