Gvaz said:
The way I think of it is:
The Relays and Citadel are stated as indestructible by any known force in the universe. I believe that there was something about a star dying a knocking some mass relay off track, which is why they didn't find it.
That isn't true. We aren't made aware of any attempts to destroy them, and we are certainly not told they are indestructible. The Mu relay in the first game was lost thanks to a supernova, but what that precisely means is rather unknown since we don't even know which star went supernova, whether it was the local star it was orbiting (in which case it would be a very impressive feat) or whether it was knocked away by interstellar debris 10 lightyears away (which is not a very impressive feat). I'm very hesitant to accept "the relays are indestructible" as a truth without something more solid to go on. Especially since Mass Effect Shields are stated to resist kinetic weapons such as slugs but not interact with radiation (i.e. the light from a giant star going supernova).
A relay surviving but being nudged off course by clouds of dust and the like from a supernova would be well within the capabilities of what we know in universe (Reapers are apparently rated to survive Dreadnought fire according to EDI circa ME2), but relays being made of metal that survives a direct supernova by magic is not. I would consider it far more parsimonious to assume the former than the latter given how vague the information we have to go on is.
The fact that the Keepers are needed to constantly maintain and rearrange parts of the Citadel seems to imply that the Citadel can be damaged at least superficially, but again, we are not aware of any attempts to say, crack it open down the middle. A station like that
should be durable by virtue of how large it is.
So, if the reapers made the relays, wouldn't they make themselves out of the same material? If so, wouldn't they also be invulnerable? They've shown themselves to be otherwise.
I would say this is a fallacy, of the "if the black box can survive plane crashes then why don't they make the plane out of it?" variety. Relays are huge, much larger than Reapers, and have a considerably larger Eezo core (that huge glowy thing vs that tiny core we saw in ME2). This handily explains why they would be tougher than Reapers (again, assuming they are, we have no way to quantify this). They also aren't required to move, which means that being constructed out of big bulky things that slow your combat maneuvering down isn't important since it's not for combat.
This is even assuming that it is indestructible, which I certainly do not.
Why was Sovereign so aside the question when asked about them?
I don't really understand what you mean by this?
Anyways it's highly plausible.
If Bioware decides that it predates the Reapers then of course it will be the case. I just don't think they've given us any reasons to believe that is the case.