Like, if I lightningbolt you, you shouldn't necissarily get +20 DB and a crit reduction.
I am agree, bonuses should be applied only once, so against attacks you apply DB, if there is no attack (crit without RR) then modify crit., and if there is crit if failed RR, apply it to RR.
2) don't apply full bonus for natural bonus (race/talent) in certain cases, as against area spells, magical defense ALWAYS is FULL applied. Maybe this 'cases' should be defined better.
Why should this be done? If you think that in comparison to the spells the racial bonuses are too high then it might be better to reduce the racial bonuses and e.g. cut them in half. But adding complexity but defining in which cases to only apply a part of the bonus would not be a good solution.
I think are not high, in a 100 based system (100 roll + 100 skill bonus possible) are not too high. The problem are rolls that are not based on 100 bonus skills, as area spells, appling full bonus for those attacks can make the resistance too high, but ONLY in that case.
If I remember well, in 'monsters & creatures' is suggested that you use half DB against area attacks, an easy method for not hard computation (search for QU, resistances, etc. for that creature).
1) double the bonus for spells.
I agree that the defensive spell should give better protection than they do at the moment. But I am quite sure that the solution to simply double the RR bonus is not a good one. It may be part of a solution, but not more.
Maybe should be based on spell roll (as roll/4 for lesser spells, roll/2 for medium spells, etc.), so more ranks in spell lists, more powerfull the spells.
I forgot to mention that we can't forget about 'spell mastery', you can increase spell power (double, triple...) the effect, so increasing the bonus could be allowed for defensive spells. This option is not provided in basic skill description or in SoHK, so we could add it.
I think it can be a good solution for ALL defensive spells and a reward for specializing in a defensive spell list (the -50 OB for bladeturn, -10 OB/10 RR fail, the bonus for 'magic defenses'). But, as many of defensive spells are instantaneous, the SCSM roll should be avoided and only use the risk of skill fumble, or in the case of using it, change the 'you can cast the next round with no roll' result for 'the spell is converted to class I' (so then initiatives are used).