Regarding Shuri, she created all the modern Black Panther costumes (nanotech, kinetic energy redirection etc), all the hand-held energy weapons and projectiles in Wakanda, the massive dome energy shield around Wakanda, the sneakers, the holographic communicators in Wakanda, the atmospheric sensors orbiting Earth that warn Wakanda of incoming threats, the magnetic levitation train in Wakanda's vibranium mines, the tech that allows cars/jets/other vehicles to be controlled remotely.... she's basically created better tech than Stark ever did before he mastered time travel. And she totally undermined both Stark and Banner with superior knowledge of Vision/the Mind Stone.

And she's still a teenager. Given time, there's absolutely no reason to believe she couldn't master time travel - her knowledge basis was far better than Stark's was when he mastered it.