FWIW I agree with @Adamant - Elven Accuracy (Wis) scratches your feat itch without delaying your primary casting ability progression, so that's clearly a solid (if not globally optimal, at least locally optimized) choice.
I disagree; optimization is fundamentally a question of the trade-offs that arise from decision points. The opportunity cost of a marginal increase in damage can often include foregone abilities that would improve overall party effectiveness, and as a result the true delta in damage is negative...
To directly answer your question, and assuming the 'shrug it off' feature is implemented as you wrote it: Any damage, including bonus dice damage from a critical hit, would only apply if the creature failed its con save. This is because "specific trumps general." The save triggers irrespective...
Arial Black is correct. But there's a logical way to represent this without natural language, too. I apologize for not using formal logic statements, but advanced forum formatting is too high-level an ability for me.
The set of elements that constitute resolving a successful attack (A) include...
I'm a big fan of augmenting paladin tankiness with sorcerer battlefield control/utility spells, so that sounds good to me. I would suggest, though, that if you choose the oath of conquest to do pal 7 / sorc 13; the level 7 conquest aura is too good to pass up for this build.