For anyone trying to complete the map, your last chance to do so is right after you defeat the three Omega Metroids. After that, you can't turn back.
That's not true at all.
You can go all the way up to the spike room outside the Queen Metroid's lair and still turn back.
Especially with the most recent patch that adds a quick-travel tube to the right of the Genetics Lab save point.
Before that, you could travel back via the waterfalls in the area just before the Genetics Lab, exactly the same as in the original Metroid 2.
Oh, and that reminds me of a gripe I have with AM2R. They ruined the tension of the genetics lab.
When you first get there in the original Metroid 2, you have a global Metroid count of 1, but when you see the egg, a stinger plays and the Metroid counter goes haywire and you suddenly have 12 more Metroids from NOWHERE.
When I was a kid, that scene was so cool.
AM2R neuters it by never giving you that false sense of nearing the end and seeing the counter go wild as it dawns on you that you're in their nest, and there are young metroids nearby.