Plumbing – Bath setup: Hand Shower and shower head on independent valves and trim

As long as the shower head inside diameter is a standard size (which I imagine it is), you really should be able to find a washer with a built-in screen at your local hardware store (although I haven't tried myself so I could be wrong).

And still if you can't find one then you can just go with a standard washer without the screen. (And maybe you could remove the screen from the broken washer and place it underneath the new standard washer that you just inserted.)

I think the screen in a shower head is not so much for aeration as to prevent contaminants from getting into the shower and interfering with the various spray patterns.

Hardly any noticeable difference; though a "high flow" showerhead would make any difference WORSE, not better, as well as increasing your water use needlessly.

The water is already traveling through many feet of pipe to get to the bathroom. Whether there's 4 more feet of pipe to the showerhead or 9 more feet of pipe to the showerhead makes very little difference, particularly at the typical 2.5 GPM flow. At a more efficient 1.5 GPM flow, even less.

4 feet of 1/2" plastic pipe @2.5 gpm = 0.625 feet added head. (0.27 PSI)

9 feet of 1/2" plastic pipe @2.5 gpm = 1.406 feet added head. (0.61 PSI)

4 feet of 1/2" plastic pipe @1.5 gpm = 0.243 feet added head. (0.11 PSI)

9 feet of 1/2" plastic pipe @1.5 gpm = 0.547 feet added head. (0.23 PSI)

There's also no particular reason to put the valve set closer to the spout - so you could just put the longer pipe on the spout, and take advantage of the pipes in the shower wall.