Tank is being the main target. etc
That's not at all what tanking is. Tanking is a term that has nothing to do with who the other team targets (well it does, but only peripherally). Tanking is your ability to take damage, ie: what a tank does in a raid. In other words, if you are a resto druid playing against a dubDPS team and you end up "tanking," that means that you are taking the brunt of the damage. The problem is that a shadow priest is NOT a good tank. You can't keep yourself up through shield + PoM + renew, meaning you have to hardcast. If you get locked out you're a dead priest and if you're hard casting flashes you're going to get kicked. Long story short, you should never be tanking. Your rogue should always be peeling 24/7 for you while you get up DoT's and do the majority of the damage. Your rogue should always get a sap off and control the DPS/other DPS. Moreover, if you're playing a rogue team your rogue should NEVER open until theirs does. You can 1v1 any other class except maybe a good frost mage, but if you're playing a frost mage team you should be dropping the mage regardless.
I'm not sure why you are recommending that he use stam gems. That's the exact opposite of what he should do. Stam gems don't give you throughput, utility, or offensive pressure. There's only been one season in which a class gemmed for stam and that was s6 Disc. Otherwise Stam is literally one of the worst stats out there for PvP because it doesn't have any defensive or offensive utility (it's just pure health).
As you said before, you have to play super offensive. You can't be super offensive if you're gemming for stamina. What BG did you play on? Most of what you say doesn't make sense or is contradictory. If you're forced into defensive play you can't take control of the game. Read what you wrote. If you get full DoTs up, then you're not in a defensive corner. If you have the ability to get full DoTs up (ie: hard casting a VT) you don't need to use fear. Moreover, you should be fearing out of blind, not the other way around because you don't want to be in range of the other teams defensive abilities (ie: if you run to the druid to fear, the warrior is going to intervene and be right on you wasting shadow step). If you fear out of blind, then the warrior can't intervene to you to gain pressure because his healer is going to visit Africa anyway. You can't really rely on vanish sapping as shadow, primarily because you're going to have to rotate burst depending on what the other healer is doing and whose head is popping up. Also, HUGE thing to remember is that you don't have glyphs that erase DoT's in this patch. Be wary of what you are DoTing and how you are DoTing.
Also, once you get full DoTs up, if you think the other healer is going to get in range (ie NS RG/SM HT) you should be MCing the target that has DoTs on it. This serves two purposes. First, it drops the healer out of combat so you can get a resap and second they can't heal their partner while your DoTs tick.
If you're forced on defense, on the other hand, your rogue is being forced to shiv and control the DPS on you. If you have to silence the healer so s/he can't run up to you to force a CD then you can't use that silence to turn the tables on them. Long story short, if you try to play defensive you're going to lose. Watch the Boozt video that you yourself linked. They can't afford to play defensive because then they fall behind by huge margins.
IE: Look at the third druid/warrior game against Grecco. The druid trinkets way too early letting the spriest not have to use silence. But if you watch, Boozt uses his blind on the druid, Tuska runs over to the druid to fear him, gets intervened, giving the druid a full GCD + movement. Had the druid not trinketed so badly, that would have been a SM and they would have lost the game.
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