Believe it or not, this is totally optional.
In the other Metroid Prime games, you have keys of some sort hidden all over the place, and you need to go looking for them at the end of the game in order to access the final boss. I’m starting to think the Valhalla isn’t an optional area, but I do know that you don’t actually need all the Energy Cells in order to reach the final area of the ship: I’ve taken several branching paths already. I’m pretty sure the only mandatory part is getting past whatever’s through that green door. With so many Energy Cells required to be retrieved as part of the main story, I don’t think you actually need to go hunting for any if you just want to finish the game, so long as you only put them in the correct sockets.
Optional or not, I do want to finally get 100% in a Metroid Prime game, and this game is one that lets me actually do that reliably with the room I keep talking about. It’s not a walkthrough, but if I recall correctly, it’s more like how you got circles on the map in Other M for wherever items are. Still, being this close to done before being told where everything is still is useful, in that the upcoming fights are that much less challenging. Veteran Mode is hard enough without handicapping myself by not exploring.