supermassive

To put this result in context, an important open question in astronomy is how supermassive black holes are formed.

The article also mentions that two supermassive black holes could orbit less than a parsec apart.

For a supermassive black hole, this could be on the order of a few seconds.

What could have prevented supermassive black holes to exist from the very start?

What if it was two supermassive blackholes rotating around center of mass?

These then began accreting material and merging with each other and gradually grew into the supermassive black holes we see today.

The theory you link to would likely not have any bearing on the issue of the existence of this supermassive black hole at such a time.

Also, this seems like a great place to ask any physicists that may be reading: Is there consensus about whether you could live inside a supermassive black hole?

It would seem quite natural that at the start there were many supermassive black holes with gas around, and so around these holes galaxies formed because of gravitational pull.

But if this theory is true, that means there should exist supermassive black holes that don't have their galaxies, or have size mismatches between galaxy and black hole (hole way too big for the size of galaxy).

For instance, how do we know that there isn't a binary supermassive black hole at the center of the Milky Way that we can't see?

As @nilkn notes, this discovery is in conflict with existing models of the formation and growth of (some) supermassive black holes, which is a reason this paper is in Nature.

The term to search for would be "supermassive black hole binaries".

To which "accepted model" are you referring – the cosmological model for the Universe, the determination of black hole masses, the formation and growth of supermassive black holes, or something else?

However, in the currently-favored cosmology, the "start" of the Universe is followed very rapidly by the onset of structure formation, so the two are effectively the same point in time (at least, for the purposes of supermassive black hole formation and growth).

I suspect that observable black holes in space retain material/energetic plasma cores similar in size to neutron stars, while "supermassive" bodies might retain cores as large as ordinary stars, but packed with the matter and energy of many ordinary stars.