They made it seem like the object was between the black holes, which would be L1, but only L4 and L5 are long-term stable. To remain at L1 (or 2 or 3), you need stationkeeping. The ability to keep station for billions of years is a wonder all on its own.
I thought the scientists just enclosed the portal in that cylinder of duranium. The final scene shows the cylinder being destroyed and the portal being exposed to space. That means the portal existed independently of the cylinder. If the cylinder was generating the portal, it would cease when the cylinder was destroyed.
That's an interesting interpretation - like Richard, I assumed the Progenitors created both the portal and the container it was in (though not necessarily that the container was generating the portal, just encasing it).
I thought the scientists from the 24th century had to have been responsible for the cylinder because they were responsible for the key that opened the cylinder. They found the Progenitor tech 800 years ago and decided it needed to be more hidden than it was. That's why they made the clues Discovery has been collecting all season. Those scientists may have followed clues left by the Progenitors themselves, but the clues Discovery has been following were left by the scientists not by the Progenitors. The clues lead to and allow the opening of the cylinder. I was thinking the portal is original to the Progenitors because it's still operational and as we saw with the Denebulan water makers, 24th century technology can fail within hundreds of years unless it's maintained.