Problem solved by putting on a new top unit. So, your guesses are likely better than mine. I agreed with rdklinc's logic that going the easier (even if less likely-seeming) route first was a good idea. I kept wondering "maybe the replacement cable is bad, or the replacement board?" and also "maybe it's another wire or connector?" Also replacing one big bunch of components at once appealed, rather than trying to go piece by piece to isolate the particular part. The top unit had a tested screen, bulb, cables, board, wires, connectors all in one package.... Just needed to plug connectors onto the logic board and see if it worked. And it did.
The top part sits a bit askew on top of the bottom. I think probably the hinge in the new screen assembly unit is slightly bent. If I can talk myself into tearing this thing down just one more time, then I wonder if I can take the hinge from my old macbook screen assembly and put it into the new without too much trouble, and see if top and bottom line up better than they're doing? It opens and closes fine, I just wish the final result of all this was more like having the Book back exactly as it was (or better) before all the trouble.
Thanks for everyone's input! It's all been valuable.