Should Any Characters Have a Previous Connection to Shepard?

IT Megathread veteran here (4000+ posts) I've probably discussed or observed every single question relating to IT there was, so I'll try to answer yours

Why are you locked into destroy with low EMS? This might be a long answer. First, "EMS" in the Indoctrination Interpretation translates, in practice, to Effective Mental Strength, as opposed to Military. They're tied together because they represent (in a flawed way since they're just numbers) Shepard's confidence. The more War Assets, the more resolve. But that's not all it means. The premise behind IT is that the Reapers want to indoctrinate Shepard, but not just turn them into a husk. They want a long-term agent, like TIM or Saren (who happen to represent the other ending options). To do that, they need to use slow, patient indoctrination. This is consistent with such oddities as Anderson dying removing 1000 EMS.

This comes with a caveat: That Shepard would be worth the effort. If they do a terrible job at uniting the galaxy, maybe they weren't that special after all (low-EMS). If they manage to get a lot of support on the other hand... (high-EMS). So what does that mean? The more Shepard proves themselves, the more options open up as vehicles for indoctrination. To what end? Depends on the sub-theory. The most obvious reason would be to use such a pivotal figure to sow as much well-placed chaos as possible. After all, the Protheans' (and presumably most others') order of battle was fractured because of separatists who wanted to control the Reapers (and probably some like Saren who willingly let themselves be "upgraded" o promote synthesis). At worst, they'd simply eliminate Shepard from the equation.

The "slow, patient indoctrination" part is key to the theory, because while the Reapers can quickly enthrall someone, they lose any uniqueness in the process. The reason Reaper Indoctrination is insidious isn't because it makes you do what the Reapers want, it's because it makes you want to - often while you still think you're in control. That's why it's More Than Mind Control. The reason it would work in the endgame is because ultimately, Shepard chooses how to move forward. Stick to your guns (literally), or try a too-good-to-be-true strategy? This sort of thing probably happened to TIM and Saren at some point, whether they remember it or not.

Anyway, another idea floating around was that they want Shepard specifically to complete this harvest's new Reaper. This goes with Reapers making one Sovereign-class Reaper per cycle out of the most threatening species and making the rest into Destroyer-class ships. So the idea is that each cycle, the Reapers also select an influential individual to serve as a "guiding intelligence" for that Reaper (à la Control ending). Last time, they wanted Javik, the Avatar of Vengeance, but he evaded them, and that's the reason there was never a Prothean Reaper. This time it's Shepard, spearhead of the resistance, Avatar of Victory. So that would fit with Shep not being valuable enough to use long-term indoctrination on in low-EMS scenarios.

All of the above ties in with the Breath Scene, not synthesis, requiring the most EMS to achieve (and therefore being the best ending by all accounts )

The last thing to remember is that the Reapers aren't letting Shepard destroy them. That option has to be there, because it's been Shepard's objective since the beginning, and this is in Shepard's head. All they can do is add alternatives. And since it's in Shep's head, they're not letting you destroy them in that sense either. Anyway, there are a lot of interpretations even under the IT umbrella. That's one. Hope it helps!

I, too, hoped beyond hope that they'd have a post-ending DLC where Shepard wakes up and you actually finish the fight. Hell, maybe that was even the plan originally, but the ending backlash forced out the EC and made the literal endings canon. Damage control. Imagine that! The fans unwittingly ruining their own ending because they cared too much. That's some serious irony. Now, I'm not saying that's what happened. But it's a thought. I agree the EC made things worse in most cases.