I don't think Disney is worried about closing down a section of the park. Remember RDCT closes down a bunch of DL. While it closes and then reopens it does shut down.
I have heard that the "Magical" fireworks show that they currently run for the summer was created to help them deal with larger summer crowds. I think they DO still have to close down a few Fantasyland attractions for this show, but not as many.
So with AP crowds, last year Disney put on what most consider to be a "better" show (RDCT) in the off-season, closing down more attractions to do so, and then to put on what many consider to be a "lesser" show in summer (WoC), keeping more attractions open. So I think that might happen here. In this case, a "better" show for the off-season might be WoC with Paradise Pier closed off and completely darkened, while a "lesser" show for summer might be WoC with Paradise Pier dimly lit and with TSMM open.
I do agree that they're hoping to keep people from viewing the show at anywhere but their optimal location. But I've never heard of modern-day Disney actually following through on forcibly preventing people from watching a show from a poor location, so long as it doesn't endanger lives or encroach on walkways. It wouldn't be hard to rope off the Hungry Bear (which is one single restaurant, not 1/4 or 1/3 of the park!) and prevent people from watching Fantasmic from there--but they don't.
In this case they may very well not allow people to watch from the "wrong" side, as some of the folks in charge reportedly want--but this would be something fairly new, as far as I know.
We'll have to wait and see what they really do--they might not even decide on a plan until they try out a soft opening or two. But given the "Nighttastic" promos, I'd be surprised (though not shocked because of the below logistics problems) if TSMM didn't stay open.
So I believe that IF TSMM IS open, then it's going to be fairly difficult to keep people from watching the show from the "wrong" side of the lagoon. Can you imagine the complaints if people tried to watch the show from a railing in a nearly-deserted area of the park where the walkways are plenty open, and then CMs came by and ordered them to move on? If they wanted to prevent people from watching the show from the pier AND TSMM is open, then I think they'd probably have to have a roped-off walkway leading both to AND from TSMM, with CMs watching it to make sure people don't "escape" to watch the show. If that were the case, people would probably end up standing still in both the entrance and exit line to watch the show, causing bottlenecks until the CMs ordered them to move on.