I like it!

You still have a chance of the fuel in the dead headed fuel rail to vapor lock.
I would have just used the stock return line and go to the regulator then return that back to the tank.Or,just return the stock return line to the tank.no need for the other regulator unless you were gonna up the HP and line size thru out the fuel system.

The biggest problem is that I could not figure out a way to straight pipe the top of the fuel rail to the bottom or return side since it is a goofy 90 degree bend where the current regulator sits but now that you mention this I could pipe it out the relief fitting on the front of the rail and send it back to the regulator if I experience issues. Will update once I run the boat in 120 degree weather here in a couple months. Hoping it doesn't happen.

The Ace, question
 

Do you have a list of what fittings you use to connect to the stock mercruiser lines? Came across your post. My VST is failed and looking to repeat what you did and keep it simple.