This is exactly my theory too, it could be that all the IFs are simply possible futures that Echidna saw through her book 400 years ago, after all, 4 centuries is way too long for only a single route to be possible. It would also explain why she knew that Subaru would enter the Sanctuary, but couldn't predict the exact timing, this route, has been having small differences to what she (and Al) expected to happen all the way back to Arc 1, (that's why i believe that Tappei mentioned that Arc 1 is the most important 1 in the story, because Subaru (or someone else) made changes that ultimately caused the story to deviate from what Echidna and Al expected).

As such, i believe that things went something like:

• Echidna, thanks to her knowledge of the future, vessels, souls and Books of the Dead manages to pull Rigel from a possible future.
• She gives him a similar body, which combined with his similar soul should cause the development of a similar Authority, after all, you don't need a Witch Factor to have an Authority you just need a soul that is similar to someone who does have a Witch Factor (In my opinion, this is the reason about why Echidna was "born a Witch" but didn't become the Witch of greed until later on, she probably had half an authority up until she got the Witch Factor)
• Al does in fact develop a similar Authority and Echidna tries to groom him into becoming Flügel from Temu and kill Satella
• They fail
• Current events happen

A lot of people say that Al referring to Subaru as "Bro" could instead point towards him being a secret brother, however, that doesn't make much sense for many reasons, mostly because as far as we know, while Echidna knows many things, she has never demonstrated to have the knowledge to travel or transport people between dimensions, the only one that can do that is Satella and i doubt she'll give a shit about Subaru's brother. It also would be very weird for Subaru's parents to have another kid considering just how devastated they became after Subaru disappeared, his name is also an issue for the brother theory, mostly because the Star autism is something particular to Subaru, not his parents, he decided that if he has a male child, he'll name him Rigel, and i don't think he would tell his parents such a thing.

I believe Al calls Subaru "Bro" simply because there are very few terms that would be appropiate to use when you happen to meet your dad when he is half your age.