Not sure why you are aggressively rambling here. First of all, FTL in Mass Effect doesn't require relays at all, so the Tempest could simply use traditional FTL at the canon speed of 12 light years per day to explore Andromeda. No adjustment to lore is necessary.
Secondly, people are upset (and I'm not one of them) because the trailer shows an apparently different kind of FTL. The reason, I think, people are upset by this is that it is pretty much confirmed at this point that the Arks leave before the ending of ME3 and the game starts when they arrive in Andromeda. So where is the time to develop new advanced tech that is completely unfamiliar to them? I'll answer it for you: there is no time to do it. So, I bet they are using Remnant FTL tech to explore Andromeda, and I have no problem with that.
Finally, yes it is confirmed that the Ark is a sleeper ship AND that the journey takes approximately 600 years to reach Andromeda, which is almost exactly the canon FTL speed of 12 light years per day. All indications are that they most certainly did NOT use newfangled technology to reach Andromeda. Now, whether they discover new tech once there and use it to their advantage - that's another story. And given that the leak is obviously true, the Remnant tech is a good bet on that.
Ok so my theory was that maybe they used wormholes, or, failing that, just use a huge amount of eezo to get there. Looks like it'll be the latter, not the former.
The galaxy is a huge place. It is not impossible for someone out there to have the smarts and the funds to stockpile eezo.
Given that Mass relays are milky way tech, and we don't even know if there is eezo in the andromeda galaxy, in MEA we will either discover new FTL tech (by that I mean utilise whatever folks use in Andromeda) or spend the game in a small cluster of systems, and not the galaxy as a whole. Either is fine.
Now, why wouldn't they tell Shepard about this brilliant plan to save our species by going to the andromeda galaxy?
Well, first of all, he doesn't need to know. So why tell him? Best case scenario is it makes no difference and Shep kills the reapers. Worst case, it creates a sense of jealousy or complacency which actively harms efforts to stop the reapers - I mean, everyone's gonna want a spot on the lifeboat. Given that Shep's mental state is fragile, it's best not to do anything which could add stress to him/her.
Also, it's not like the reapers have mind control abilities oh wait they do better keep the whole save the species stuff on the down low then. Indoctrination amongst the ARK workers is absolutely the worst case scenario as it's a guarantee that it'll be sabotaged.