43 Post - new owner question #1

You're mixing intakes and outflows in your questions. Let's go to your black water (what goes out of your toilet). You have a Y valve atop your holding tank. In one direction it sends it overboard, and in the other it goes to your holding tank. In most areas you must keep that turned to the tank side and cable tied in that position preventing overboard discharge. f you're out at sea and your holding tank is full you can switch it and discharge overboard or use your macerator to pump your tank overboard, but your dock neighbors won't be happy if you go overboard in local waters.
With the intakes you really need to trace them to their end so you know what to shut off if you get a leak. I'm a little surprised to hear of PVC being used, but would guess that may have gone to or for a live well, possible your salt water wash down. And yes, anything in use needs to have the seacock open (motors, gen, saltwater head, A/C.
Re: The Admin's post, nobody searches for 43' Post unless they're looking for general info on a 43' Post. They'll search for specific issues, i.e. holding tank, through hulls, black water, etc. So titling your posts that way will make them much more useful for people searching for answers in the future.

Good thought with the crash valve, but I'd want the end visible. God forbid you use it and it runs dry.

One of my first projects was to abandon the raw water intake and switch over to a fresh water flush system. Cut my holding tank odor issues down to almost nil.

The raw water seacock on my starboard engine is Td as well, on mine it feeds my AC units.

Switching to a freshwater toilet can be expensive, but IMHO well worth it. Easier to maintain and yes, gets rid of the stink.