Wait, whole sense card? Does it count as one pin, or it's just "some copper fill" for diptrace?
One can also electrically connect the top pad to the line, using a via. That will result in 3-4x better SNR. This also means the pad under the ex-floating pad is not needed. It will probably be beneficial to put the ground plane under both pads, but I didn't experiment with that.
4-layer PCB surely allows for awesome things like "frameless" layouts and can have an integrated ground plane (which enables whole new worlds in "rest of the keyboard" design - you can make everything from transparent acrylic, for example), but we don't yet have data on 4-layer PCB longevity in curved-plate designs.
Sem-related - my experiments with pad layouts had shown that only bottom 10mm of the pad are critical for model F - the rest can be freely used for any other purposes. You MUST keep top "dots" of the footprint - they lift the flipper's "legs" to ensure proper contact area when the flipper is down - but you can run traces thru those (ANY traces - rows, columns, ground, LEDs - whatever), because those dots are too far to interfere with sensing.