Radio frequency interference, at 1.5mc there most likely wont be any affect on your setup as it will not respond to freq's that high, harmonics even will most likely be out of reach.
Yes that schem looks right.
Electrolytic s will be fine.
Solder the grounds together and to the ground point on one of the audio in sockets.
Ok maybe I didnt explain that well.
Solder the shields togther then solder a black wire or whatever colour you like to them and lead that over to the grounds of one of the decks audio connectors & solder it on.
The layout may be optimised so the ground at that point is quiet or a slightly different level than elsewhere, so using that point ought to offer the least issues.