As the OP stated, PvP set bonuses are necessary for the best gear on various classes. Due to this issue, players from the time of his post going forward are stuck at a disadvantage compared to those who started earlier.
More importantly, the need for PvE players to do PvP in order to get the correct shells/armorings is unnecessary in the first place. Many players dislike PvP, which creates problems of its own if they feel forced into doing so just to get what they need in PvE. A similar problem occurred with Relics where the best ones required PvP and fixing it here would help the community as it did there. To resolve this issue, I have two possible solutions.
1) Allows players the ability to change between set bonuses
I don't know the difficulty of this, but if it's possible, this would probably be the optimal solution. Essentially, players would be able to choose which set bonus they want from their gear through an in-game option (like a clickable toggle) once they have any of the current shells/armorings that provide them. This allows players the ability to quickly experiment and mix various bonuses for their class and gives them the freedom to wear any shells they prefer for the sake of appearance.
2) Make Recruit gear give out PvP bonuses
Players reaching 50 now get almost a complete set of Tionese gear, which gives them the PvE set bonuses for their class, but the PvP Recruit gear does not. Since Recruit gear can be purchased with credits, this allows players to buy the shell to use for the set bonus. As noted by the OP, though, Campaign and Dread Guard armorings do take precedence so players would need to use crafted armorings to fill those slots. Another disadvantage of this is that players who want to optimize their gear would have to wear Recruit gear, while I think many people would like to use whatever shells they prefer for a particular appearance.