Summoning Skill and conversion: How does it work exactely?

[Alhanelem]

100% sure about this?

I'm pretty sure about this. Why would you have any reason to think otherwise? There would be zero reason to have any summoning magic skill + if you weren't at the level cap if it weren't the case. If you're level 60 and your skill is capped for level 60 and you have + skill, you should be getting increased buff durations. Testable in ballista- if you get the benefit below level 75, then

Do you put an item with +19 SMN skill or one with +10 Macc in a certain BP set?

Skill

A SMN Skill +8 or a Pet Macc+7?

Skill

I've never had an issue with magic accuracy. It seems to be naturally better than physical accuracy, as I haven't been having appropriate-element magic BPs resisting more than physical BPs miss. I try to avoid having to carry around more gear than really necessary when the benefit is likely imperceptible. (Though, the mog wardrobe may change this for me)

Keep in mind, short of doing thousands of samples with both +acc and no +acc, it's difficult to conclusively say here, because /checkparam can't tell you if an effect that only applies during a BP is applied. (and I just realized, that checkparam doesn't show magic accuracy anyway)

And if it does, does it work for BP: Ward debuffs or just for BP: Rage?

If it does, it would affect anything that targets an enemy. (Note that damaging pacts under BP Ward (e.g. diabolos) are still affected by the accuracy and skill as far as I can tell).

Duration of debuffs is testable in ballista, as well. I'd be lying if I said I tested that, but I have not ever heard of anyone reporting increased debuff durations from BP wards.

[Zhronne]

I'm pretty sure about this. Why would you have any reason to think otherwise?

Because of the example I wrote in my post of course
Going by Wiki's and BG's tests + description, each skill over 300 (no matter if skill is capped or not) adds +1 second of duration to BP: Ward buffs, capping at 500 skill for +200 seconds (3mins 20 seconds).
Going according to this the threshold seems to be 300 skill, not your current skill cap.

I haven't been having appropriate-element magic BPs resisting more than physical BPs miss

It was more a matter of the Debuff BP Wards of course, rather than the magical BP Rage. Hardly any SMN with a bit of brains would have serious issues with that, unless I'm being too optimistic.

Keep in mind, short of doing thousands of samples with both +acc and no +acc, it's difficult to conclusively say here

Agreed, but non-conclusive tests are better than no tests at all.
Some comments concerning SMN actually converting to Macc can be found Here and Here.

If it does, it would affect anything that targets an enemy.

Yes, that was my theory/hunch as well, but I never did tests myself.
Appearently it's very likely that things are really like this so I can be only happy about it

Now the questions remains about SMN skill vs Macc in slots which offer both options, but without a conversion ratio or without VERY extensive and unrealistically practicable tests, we'll never know the answer to this.
Altough I feel it's a less important question now that we know that, whatever the ratio, Skill does indeed convert to Macc.

Duration of debuffs is testable in ballista, as well.

Not sure if Skill affects debuff duration as well (might be). Would be hard to test because I think (???) that debuffs, unlike buffs, do not have a fixed duration, but a dynamic one which changes according to pet level/stats and target level/stats/resistances.

Nevermind.
SMN skill DOES affect duration, and the majority of SMN debuffs have a fixed duration no matter levels, resistances and whatever else.