A volt turn team is nothing more than a very fluid hyper offense team. This means that it strives to achieve constant offensive domination throughout the match (brought about by always having momentum).
Having a straight up dual-wall defensive core as part of this strategy is doing you no favours - it not only kills your momentum but also takes up valuable slots that could be used for providing more offensive synergy and coverage (thereby allowing you to handle things like Scarf Terrakion). I understand if you want defensive measures to fall back on comfortably, but limit that kinda thing to all-purpose pivots (Porygon2) or typing resistances (throwing in some basic defensive synergy).
Scizor is a premier U-turner, has great synergy with Rotom W and Salamence, and provides the famous Bullet Punch (which will kill Terrakion for you). It also helps you against Reuniclus, which can set-up trick room and really screw over your team.
Jirachi and Celebi can put down SR, use U-turn, and be generally unpredictable with their vast movepools. They also have decent bulk so you won't feel the loss of your Skarmory + Blissey Combo.
A scarfed Terrakion of your own is useful in tight situations as well as sun teams.
I'm also not too sure about having Magnezone. It may help Salamence sweep, but the goal of Volt-Turn is to constantly whittle down enemy HP (in many cases so that something, such as Scarf Salamence, may clean up later on).