Even if they could (it’s unlikely) … would they? Philea was a pretty risky and relatively small add-on to the main Rosetta mission. Rosetta does important stuff all on its own, it’s not just a vehicle to get Philae there and relay data.

Rosetta will stay with the comet as it gets closer to the sun and observe it.

Even under perfect conditions, this would provide a tiny bit of extra light for a few seconds per orbit, not even close to enough to make a difference unfortunately.

This is probably not feasible, but in order for reflection to help much Rosetta would have to be much farther from the comet, so that its orbit would be comet-synchronous. Actually I doubt Rosetta has any surface reflective enough to help with this anyway. If it were used this way it would probably not be pointing its transmitter toward Earth.