Arreats, Enigma and Highlords is enough to give 55% hork which is good enough really. If you want to go the 105FCR route and use Arachnid's mesh (+10%FCR ring), then your skill shout is enough to give 56% hork (with double HOTO). I might be undervaluing metalgrid a bit, but levels and sharp charms cures the need for its AR. And if you wanted it for the resistances, mara's would probably be a better choice; still even, then I would also recommend highlord's and finding res elsewhere (such as charms).
I would third(fourth?) the drop the skillers plan, in particular, for your best Sharp charms. I use sharp charms that give life and strength as a secondary affix mostly, though one or two gives a massive chunk of lightning damage (which is mostly useful for giving a visual queue as to the range of WW, and a negligible boost to zerking down PIs).
As to the socket you asked about, I put a Diamond Jewel in my arreat's for yet more damage, but um or fire/cold resist is a good choice too if you're feeling your resists are lacking. In my case, fire resist is the only resist I over buffed (to keep the pretty much a non-issue even under conviction), while getting what cold I could to avoid issues with cold death explosions potentially taking me out. I ended up with good LR by accident, but it's occassionally helpful when something spawns PI and is Lightning enchanted.
For my merc I went with Reapers(Shael), Kira's(15% IAS + Fire Resist), and Guardian Angel(15% IAS + AR). I tried other armours like Treachery and Gladiator's Bane, but I found they allowed the merc to get himself into trouble more often, and I didn't really want to babysit him while running so I just went with the tried and true Kiras+GA combo. The fire resist portion of the jewels is mostly to help overbuff his FR for conviction packs.