First let me condense this a bit for presentation purposes; then a little markup:

That one wire (arrow) is a good place to add extra filtering if needed.
You may also put a CMC at the input jack, in case your ground is dirty and a choke is needed. The power brick should be okay (shouldn't be a source of noise or surge), but it would help with that too.
The important thing is this: if the loads are not common ground (there is no sneak path around the CMC), then the load must be returned to the X'd line. (Or even if they are, if you want to ensure load current flows through the cable only and not through sneak paths, this must be done. Not very relevant at 300mA, aside from the CMC, but this can be important for high current loads where the return current causes significant voltage drops across the ground.)
In that case, the CMC would be relevant, and there would be no ground bridge under U2. Large caps to ground (C5, C7) probably wouldn't be a problem, but probably wouldn't be necessary either. These would typically be ~nF. Diff filtering (C10, C11) should be larger, 2.2uF say; the values shown won't really do anything, and C11 (3.3nF) with the ~10uH leakage inductance will lead to a transmission peak at ~880kHz, which may happen to fall near a harmonic (depending on if the DC-DC is variable frequency or relatively stable).
But with everything common ground, there's no common mode, just make sure the ground is solid under it, and bring out the connections to the same side so that C8 and C10 are somewhat nearby (and again, C10 and C11 should be larger values, and the CMC can be replaced with a single 1uH inductor on the 12V line).
Cheers,
Tim