Summoning Skill and conversion: How does it work exactely?

[Alhanelem]

It mentions "skill which exceeds skill cap". Does this mean the current cap (417) or the cap back then at 75? (269)

It's the cap for your level.

Does this imply that skill below cap, whichever it is, is still useless as it was before 2006?

if your skill is below the cap it is still used to determine Elemental Siphon potency and elemental spirit casting frequency. (as well as spell interruption rate when casting spells from that family, as with all magic skills)

There's a talk of "increased accuracy for BP: Rage". Is it a generic omnicomprehensive accuracy? Or just physical accuracy?

Fairly sure its physical accuracy only but not 100% sure on that. would require more testing than I've done.

It says "increased duration for BP: Ward". It's pretty obvious for buff Wards, but what about debuff Wards? Do they get increased duration too?

Only affects buffs (including Perfect Defense). When Perfect Defense was initially nerfed, all the summon magic skill + in the game couldn't get you to the maximum PD duration, not sure if that's still true or not (haven't kept up to date on new gear)

There is no reason not to have more summoning magic. You should have as much of it as possible, so knowing more about this shouldn't affect your gear planning.

[Zhronne]

It's the cap for your level.

100% sure about this?
Pretty sure my mule level 99 with 320ish uncapped SMN skill was getting over 3mins hastega.
Don't have access to it anymore so I can't double check.

(haven't kept up to date on new gear)

417 base skill +16 merits +127 from gear = 560.
Would need 40 more to reach the previous cap of PD (SE mentioned needing 600 SMN skill)
Of course more SMN skill gear in the future I bet, I went with the currently available BiS options.

There is no reason not to have more summoning magic.

I never remotely implied this.
I'm just looking to find out how/if SMN skill affects BPs and how, other than what's already known.
Namely does SMN convert to Pet Macc? And if it does, does it work for BP: Ward debuffs or just for BP: Rage?

so knowing more about this shouldn't affect your gear planning.

It so does.
Do you put an item with +19 SMN skill or one with +10 Macc in a certain BP set?
A SMN Skill +8 or a Pet Macc+7?
If Skill indeed converts to macc for both BPs it opens a lot of possibilities, whereas if it doesn't then it just doesn't matter wether you equip it or not during those BPs.