It's a nice theory, but apparently, no. Or at least, it certainly wasn't a conscious, deliberate pre-planned masterplan. According to an interview with co-writer Gillian Flynn:
"I don't believe she knew how it was going to turn out, but I do think that she knew that [Harry turning up and trying to take the money] was going to happen"
So she hadn't formulated a specific plan of what she'd do if Harry showed up. Which makes sense - she was in a vulnerable position and didn't look prepared when Harry did show up.
The journalist who wrote up that interview gives the following interpretation of Veronica's thoughts after the scene in Amanda's apartment. Obviously this isn't 100% official, but it's based on a conversation with the writer, so it's likely to be at least close to the mark:
After leaving Amanda’s apartment, Veronica never tells anyone that Harry may still be alive, indicating that a big part of her wants to believe he would never betray her that way. But Flynn said that Veronica knows she and Harry will eventually meet again.
“It’s a love story,” she said. “If love is truly knowing your partner, that agreement has to go all the way through. You have to truly agree to know your partner, warts and all.” But as we see, Veronica had been content to enjoy the life Harry provided her without really interrogating how he provided it... But as Veronica begins to use Harry’s notebook... she exposes herself for the first time to his world’s ruthlessness and lethal menace
Flynn then describes Veronica and Harry's final confrontation as:
"...the two of them truly meeting for the first time — in an ugly way, but in a true way at least, finally"
So, throughout the film, Veronica was starting to see and come to terms with just how much of a villain Harry had been, and how much he had betrayed her, but there was still some lingering confusion and denial.
It fits a general theme of the film, that Veronica didn't press a button and magically transform from grieving widow and union rep to hardened, efficient criminal genius seeking revenge.
It was a slow, painful toughening-up process, full of mistakes and lingering naivety.
...[The film is] honest about how hard it would be for four women with zero experience to pull off a heist this dangerous and elaborate... “We wanted them to make it through in an ugly way.”
It wouldn't be in character for Veronica to see Linda panic and shoot Tom Mulligan, and immediately, coldly, clinically think "Aha, this is an opportunity for me to pin this killing on Harry, when I kill him later when he shows up, which I know he will do. I just need to take this specific gun, right now".
So why did she take the gun?
First, here's that article's presumably-correct version of what actually happened in that fast moving scene:
Alice is shot trying to grab Tom’s gun, causing Linda to shoot and kill Tom
There are two far simpler explanations that perfectly fit her character at this point:
- She confiscates the gun because she doesn't trust Linda and Alice not to make another mistake. She instinctively snatches the gun out of Linda's hand while she's still in shock - exactly the kind of instinctive response you'd expect from someone who sees themselves as being in charge of the situation, who has spent a lot of time responsible for children and teenagers (background in teaching, plus raising Marcus), and sees someone with something dangerous they can't be trusted to handle responsibly. It wasn't the serious-toned "Now give me the gun" of a criminal executing a secret plan, it was more like snatching a box of matches from a child.
- She keeps that particular gun because it would be out of character for her to trust anyone but herself to ensure that the gun that can be linked to Tom Mulligan's shooting is disposed of adequately (which, of course, in the end she does by seeing the opportunity to plant it on Harry - after taking a long moment to pull herself together emotionally, and assess the situation and the risks).
In the above-linked interview, Flynn talks a little about how she sees there being something of a mother-daughter-like relationship between Veronica and the others, and of course Veronica has continually been exasperated by their lack of criminal competence. She barely trusted them to not screw everything up, even before Alice got herself shot and Linda killed Tom Mulligan.
She's doing what she does throughout the film - taking charge of the situation, assessing the risks, and trying to protect everyone from their own blundering.