1. Sir, you might be referring to fire immunity. It's different than fire resistant. The Germas is still flinched by fire enveloping them, which means RS isn't fire immune.
2. So if we're talking about sense, how about this. If you can lit your legs on fire, but your clothing is resisting the fire itself, wouldn't it reduce the fire potency? Because instead of just flaming normally, the clothing acts as anti-fire. The embers won't be as hot because it's being contradicted instead.
3. Yes, because Sanji has mastery over it. His legs is already moves so fast, we don't see the frictions anymore. Maybe just a simple gesture can ignite the fire.
What I'm implying is Sanji's fire is science based, it leans more on scientific reasons than just "magic" haki. Which means if the RS is fire resistant itself, it would act as repellant to his DJ.