Will a slightly warped desk ruin my extended mousepad?

Probably not. Most modern optical and laser sensors handle a tiny bit of unevenness just fine. A thick cloth pad will conform to a visible dip or bump, and even a thin hard pad won’t cause pointer issues unless the warp lifts a corner off the surface entirely. The bigger risk is the pad sliding around, but that’s a grip issue, not a performance one. Don’t buy a new desk just for this.