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.