Anyhow, the point is, there will be no "tunneling into another universe", no visitors "from another dimension" etc.,
EVEN IF THEY EXIST
So FAPP, fuggedaboudit.
The formalism has never allowed for that possibility. It happens only in sci-fi.
This appears to be a lay translation of the formal disallowance.
Not sure what you mean. The many worlds interpretation has always ruled out, by its very structure or formalism, any form of contact between these branching worlds. That is not a “lay translation,” that is just how the metatheory goes.
SciFi frequently uses a similar concept, in which the act of time travel necessarily causes (possibly, but not necessarily, by means of selecting different quantum choices) the formation of another timeline (or perhaps moves the time traveller(s) into such a timeline that already existed).
While this is superficially much the same as the MWI, there is no reason to believe it is the same thing (unless the Word of God - ie, the author of a given work - explicitly says it is).
Back to the Future certainly does this; Marty McFly changes the past, moves forward to a new future caused by the changes, and then goes back and "fixes" the past, and moves forward yet again to yet another new future. The dystopian world where Biff runs casinos and has become the ruler of everything still exists; By "fixing" things so that Biff doesn't get the Sports Almanac after all, Marty saves
himself by moving into a future in which Biff never got the Almanac, but does nothing for anyone else trapped in the dystopian future, which apparently still exists alongside a number of "happy ending" futures.
My point being that, as this is all speculation, anything goes. We can write whatever stories we like. If QM rules out something we would like to include in our narrative, we can just invent a MacGuffin that allows us to travel to an alternate leg of the trousers of spacetime, where QM doesn't rule out that plot device.
Sure, the MWI metatheory may not allow it; But that just means we need another metatheory that does.
Fiction is fun. It's probably not real, though. Probably.