1 : Still not corsaire c3, so i can’t answer you properly. I guess it’s not worth till max rank is capped at 7. But it depends on your build 
2 : Yes it does, I don’t have numbers (it’s not on any database i know for the moment) but it’s written on the tooltip of the skill. From 5 to 10, I didn’t see a huge difference in stealing iterations, but I had the sensation of dropping more items instead of silver. Nothing concrete anyway, since you’ll double the drop rate from stealing while you grind with Double weapon assault (cafri sun + sacramente = 2 additional hit on each hand, you do 6 hit per auto-attack).
3 : Right now, i’m playing with cafrisun, it adds additional damage (works great with arde dagger fire property damage and blessings), it’s leather. But I’ve raised plate to level 20, because I think i’ll have to change for plate in few levels (cafrisun doesn’t bring you any defense since it’s an armor level 15). I’d say, go on plate, it upgrades your stamina and makes you tankier vs melee damages (you need it if you’re not dex based build). If you planed to rely a lot of evasion (I don’t see why you’d go full dex, but well, maybe it’s used in other builds) leather is what you will need since it raises your evasion.
4 : In fact, corsaire is good in almost every build. For pvp purpose, it’s never a bad thing to take a rank of corsaire for CC, and for grinding, jolly rogger is a huge boost. Both of most effective corsaire builds are based on hoplite atm (hexxen dropper at corsair c2 is a huge pierce burst, going well with hoplite pierce build). With the incoming buff of barbarians, i guess it can be a good combo based on slash damages. Atm, barbarians > corsair is already decent. Barbarian brings you a lot of stuns, and frenzy/warcry are considerable boosts.
I’m only 190, not a swordie main player, I fear the 200+ 