After being stuck for a bit unable to figure out where the heck I was supposed to be going, I finally stumbled across the security robot I fought earlier. Only this time, he had rockets and could electrify the water he was wading around in.
For some reason, though, he was easier to kill. I got the Wave Beam from him, so I can now shoot through walls. I then went into the Restricted Area the robot was trying to prevent me from entering, to find a load of Metroids… even though they were supposedly extinct. Obviously not. The SA-X appeared, but thankfully took more interest in the Metroids than in me. I escaped, and worked my way back to Sector 1.
There, I found Ridley. Well, Ridley-X (or something) anyway. He/she/it was easy enough to kill, although I messed up my first attempt. It left behind my Screw Attack, which I’m pretty sure is the last weapon I need before finishing the game. I then worked my way up (as instructed) to the Observation Deck, where I met another (oh yes, there’s more than one) SA-X. Whom it seems I now have to kill. And died. Yet again.