my shower sump has a large loop that goes above the waterline to prevent back flooding. also, in all honest, i never have the head sink seacock open. if i need it for the shower i open it then and reclose it immediately when done.
i don't use the head sink, i brush teeth in the galley sink, wash hands in galley sink, etc etc etc. the head sink gets zero use. i would remove the head sink if i didn't think it would hurt resale value.
Also, a concern of yours about the Tee being a failure point. When i said mine was tee'd i wasn't being exactly truthfull. It's not a simple Tee fitting. I have a 3-way ball valve mounted atop the seacock. with the 3way ball valve in position 1 the sink drains to the seacock. in position 2 the shower sump drain to the seacock. No two can be lined up at the same time.
Its not the best system out there, but it isolates one drain from the other, and if there is a problem with both, i can just isolate the whole damn thing with the seacock.