If Walder had been more loyal / Stevron had been Lord of the Crossing prior to the Lannister invasion of the Riverlands, do you think the Frey forces being present at the first battle of Riverrun would have made any difference? Obviously there would be massive butterflies long term, but could the Riverlords have won that battle with an extra ~4,000 men?

Probably not. My speculation is that First Riverrun was a surprise attack, where Jaime caught the levies unprepared as Edmure was expecting Vance and Piper to hold Jaime off at the Golden Tooth while he gathered and drilled his forces. 

In that situation, numbers aren’t hugely important – as we see at Oxcross. Armies really don’t like getting flanked or hit from the rear, and there’s lots of historical examples of much larger armies getting beaten by smaller armies who managed to attack from an unexpected direction.