>4) Does the effectiveness of SVS (and other saves) depend on level? For example, while -20 may do very little at hero, is it good at lower ranks?


I don't know all the mechanics to this question but I do know there is a confounding effect: levels and spellcraft.

Higher spellcraft skills means that the target saves less often, and this is more likely to occur at the character casts many more hours of spells over a lifetime. This will feel like more saves are required.