The simple answer, for the player themselves at least, is that it's entirely based on the stats on the gear.
The con of the mob will change based on your item level, and Trusts will scale according to it. You're just getting the benefits of the listed stats.
Additionally, main/sub/ranged only takes into account your highest ilevel item in those slots. So for your example there, if you're wearing an i119 axe (and armor) then your item level is still i119, despite what your shield/bow are.
For most weapons, the only defensive aspect you might lose is the parry/guard/whatever bonus, which wouldn't tend to be applicable to pulling (need to be engaged, or have things in front, etc).
The reverse is also true, where you can have armor (Argosy for instance) that's i119, but offers little of the normal defensive benefits, effectively leaving you vulnerable compared to other options.
Also consider that the cumulative Gifts on a Mastered job can often give a significant bump to defenses, but are not reflected at all in the item level rating.
In addition, high level areas such as Adoulin and Escha areas, don't do level correction (at least, they don't do it for us attacking mobs).
It's only usually important to focus on that item level number if trying to keep Trusts at their potential.