You can only access objects on the running layout. Objects on other layouts don't exist so you can't access them.
Maybe you can make the objects global and just hide them most of the time. Or just storing the health in something global.
Yes, making the Block HPs global would work. I wonder if this would become an issue down the line if I'm making a full game and there are essentially over 100 Block HPs having their own global events. Can I keep a list of globals on its own event sheet and have them work properly?
Also, could I use multiple layers to accomplish this? I'm guessing I can make certain layers inactive when I'm on another. Guess I'm going to need the full version eventually to get some of this stuff working.