[iRO or kRO Mechanics?]

I think it's actually quite simple.

Every time it's possible, we should follow kRO, in other case follow iRO. To be precise, if we have exact and complete info how something works in kRO, just do it this way. If we don't have enough info, it's better to have iRO version of a feature implemented (hopefully temporarily until kRO data is gathered) than nothing at all.

I agree completely, Koreans know their mechanics, % > Def/Mdef tweaks any day.

[Extra Item Bonuses (Expansion Pack)]

Adding "new item bonuses" or "custom bonuses" to the source sounds like a horrible idea.

If the item bonuses are custom then they should not be implemented into rathena since that could have balance issues. Anyone who wants the customs in their server should have the option to add it at their own discretion imo.