If your largest skillet isn't large enough to hold 12 oz. pasta and toss everything together, dump the mixture back into the pot from cooking the rigatoni and toss.
The Italian word for the ideal risotto texture is all'onda-literally, "like a wave." Risotto should spread and move and undulate. If you can stand a spoon up in it, it needs more liquid.
The reason this salad holds up so well at room temperature is because you dress it twice. It absorbs the first round of dressing completely, the second addition keeps it glossy, and a fistful of walnuts...