Betty's advance showed she was looking out for his well-being. The flirty looks and smiles showed attraction. The advance also shows that there was a level of trust between the two of them.
Yeah and? How is this an important scene? What is so important about it? How is Betty's role in the three movies more important than Gwen's? Your response here doesn't justify your belief at all.
For the purpose of making Mary Jane jealous (and nothing else of any importance at all)
Incorrect. Spider-Man saving her led to Spider-Man's celebration by the city, which inflated Spidey's ego, made MJ feel worse about her failing career, and led to the kiss between Gwen and Spidey, which caused friction between Peter and MJ. Knowing that MJ hated that he kissed Gwen, he later deliberately took Gwen to MJ's Jazz club, which Eddie Brock saw, and later kidnapped MJ because of that.
It was never just about making MJ jealous. There was all kinds of repercussions to Gwen being in this movie. She may have been a plot device, but she contributed more to the movie than Betty did in all three put together.
So your claim Betty was important is factually false.
Raimi could have just made up a name and filled that role as well.
Yeah if you ignore she was also a Science major, a model, and the daughter of Captain Stacy, too.
And I don't think I even need to get started on the lazy writing of Eddie and Gwen, two characters who have had nothing to do with one another in comics, forming a toss-away relationship.
They had no relationship at all. That was the point. It was an obsession Eddie had with her. A more believable obsession I might had than he ever had with Spidey in the comics since he had an actual connection with Gwen. He equated a coffee they had once as an amazing night together, and Gwen rightly laughed that off.
Betty would have been just fine for that role, too. Gwen is far too important to the Spidey mythos to have been so disposable in SM3.
Yeah Betty could have done the role, too, but she wouldn't have fared any better. We didn't know anything about Betty in these movies. All we knew was she was Jonah's secretary. With Gwen we knew she was a Science major, a model on the side, she collected pictures for her portfolio, and she was the daughter of the Police Captain.
That's ten times more insight than we had on movie Betty Brant. So saying Betty could have filled in Gwen's role isn't saying much.