I love your creations and you are way ahead of where I was when I first started. What's neat about tombstone building is that every year the bar get raised higher and then they show how it done. That, in turn helps others build on the knowledge learned. Haunters are the most amazing people.
Many times I use acrylic paint (artists' paints) for details after it was base coated with Drylok. Because you used exterior latex paint that would give it the same protection so you are good there. I've never had problem with the details paint effects washing away so you shouldn't be concerned. You are good to go :)
Not sure but if you strike out with Home Depot/Lowe's perhaps try Ace Hardware. They always seem to surprise me that they carry stuff I don't usually see at other places.
I like the water-proofing characteristics of Drylok because I'm usually adding in details of things that were dipped in monster mud so I need it to be waterproofed. The pink foam itself doesn't need to be waterproofed. It's that way on it's own. I love Drylok not only for the water-proofing though. That grit does give it...