If Yelena was around during Iron Man 2, she still wouldn't prop Tony
This is creepy, because if Yelena was an operative during Iron Man 1 or 2, she would be barely legal (if she even is legal). Sad to say, it's entirely possible Tony would still lust after her. In this gem, Howard told Tony about Black Widows, even though we know Howard NEVER trusted Tony with anything, particularly Shield and its secrets.
So what would REALLY happen if a teenage Yelena was sent to get close to Tony?
***
Yelena shuddered in revulsion as Stark's hand landed on her thigh. He had gradually moved his chair closer to hers throughout the presentation by Potts, the soon-to-be CEO of Stark Industries and incidentally Stark's lover.
Stark was such a bourgeois capitalist pig, emphasis on pig.
She closed her eyes before she rolled them. Her orders were to get close to Stark and steal the secrets of the Iron Man armor and arc reactor. Yelena had already had to pretend to find Stark attractive and amazing to get this job. She wasn't sure how much farther she would have to go. But she would do what she had to, no matter how disgusting.
***
After a week as Stark's "very personal assistant," under Stark's desk, Yelena actually felt sorry for Potts, who supposedly was in love with the creep.
She was clapping at Stark's laughable attempt at boxing when she froze. Potts walked in with a VERY familiar companion.
Natasha--HERE. The former Black Widow didn't betray any surprise at seeing Yelena. Yelena managed to keep her own expression neutral.
She even hid her annoyance when Stark added the redhead to his personal staff.
What she could not hide was the surprise when Romanov showed up at Yelena's scheduled rendezvous with her handler.
Romanov sat down with a coffee and said, "He's not going to show."
Yelena couldn't say she was disappointed. Her handler was as bad as Stark--old, smelly, and expecting "personal service." She leaned back and asked, "So what do you plan to do?"
Romanov looked at Yelena a long moment, then gave a low whistle.
Yelena felt compelled to respond.
Romanov nodded and said, "Seeing you...it's shifted priorities for me. I'm changing my mission from protecting Stark to rescuing our sisters and destroying the Red Room forever. Want to join me?"
Yelena shivered with the idea of turning on everything and everyone she'd known. "What is your plan?"
***
Yelena and Rom--Natasha made as good a team as they always did. With the help of Alexei Shostakov, Melina Vostokoff, and the archer Hawkeye, they were able to do everything they planned. Most of the Black Widows retired to find very different lives. Yelena joined Natasha in Shield.
They persuaded Justin Hammer that taking two gorgeous ladies on a tour of Monaco was more fun than trying to get back at Stark, so Vanko was never broken out of prison.
***
Yelena never went near Stark again, until the Chitauri Incident. When Yelena refused to go to eat shwarma afterward, she admitted to Natasha and the others what she'd been forced to do to get her position at Stark Industries when she was under cover.
When the other Avengers found out, they broke ties with Tony. Yelena, Natasha, Clint, and Steve found themselves forming a little family group as they worked for Shield. Clint soon brought them home to introduce the newcomers to Laura and the kids.
After Hydra was revealed, they worked with Sam, Bruce, Thor, Rhodey, Bucky, Nick, and Maria to take down the remaining Hydra bases. Without Tony in the mix, the scepter was recovered and Strucker arrested without incident. Wanda and Pietro joined the team.
There was no Sokovia disaster, no Sokovia Accords, no split team when the Black Order arrived. Dr. Strange was brought to Wakanda to be protected and the heroes (including the Hulk) were able to take out Thanos. Wanda destroyed the Mind Stone, which prevented anyone from ever trying to gather the Stones again.
As for Tony Stark, he learned a painful lesson in why he shouldn't exploit his employees. The #MeToo movement came a LOT sooner in this universe when Tony Stark got canceled.