In your opinion, if Stevron Frey was Lord of the Crossing at the beginning of the WOTFK, do you think he would have joined Edmure’s initial war effort against the Lannisters? Or would he still hole up in the Twins and demand Catelyn/Robb Stark concessions for Frey support?
I haven't really done the close-up on Stevron Frey, so, keep in mind I'll be speculating based on my own speculation.
Working with Lord Stevron rather than Lord Walder would be much more straightforward for the Starks, but that doesn't mean they'd have Lord Stevron's unconditional support. Stevron is a cautious man, and he wants to take care of his family. Moreover, I think Stevron has some wits about him, and he understands a) how much the Starks need his family's support, and b) how risky it is to join in with the rebels against the Lannister regime.
He'd want to have some discussion with Robb before choosing a side. I think the discussion would center more on how much they each stand to gain/lose from the war, rather than his multiple decades of resentments against various lords and their looking down on him, but there would be a discussion because Lord Stevron would want some assurances.
Regardless of what form those assurances would take, I doubt that Lord Stevron would respond to a betrayal by using a wedding as the site of a massacre. Much less his own daughter's wedding. He'd still want to see some investment from the Starks. As a cautious man looking after his family, I think Stevron would be less interested in what he's wanted from the Paramounts for decades already, and more interested in what a Stark-Tully victory would mean for the Freys in the long term. (Not to mention what a Lannister victory would mean for the Freys.) If I were Lord Stevron---and this is where my speculation is likely turning to projection rather than analysis---I would want this energetic, charismatic 15-year-old boy to show his work. I'd want to know how well he understands that the Riverlands are surrounded by nearly all the other kingdoms and they will still be surrounded regardless of who wins.
As this discussion is taking place before King Robert dies, before Ned is executed, before Robb is declared King in the North, before Stannis and Renly make their bids for the throne, etc, there aren't nearly so many moving parts as later on and what they're trying to accomplish isn't nearly so big. But still. I think Lord Stevron would have some conditions for his support.