Why do you think House Mallister’s forces were still intact following the initial Lannister victory at Riverrun? Did they not participate in that battle? If so, why?

No, they probably participated, because otherwise (as with the Freys) it would have been mentioned.

Honestly, it was probably a matter of luck – having a commander who kept their cool, being placed on the right end of the line so that the rest of the army was between them and the Lannisters, sending a contingent that was mostly cavalry so it was easier to get away, etc.