Here's a condensed version of the above charts, showing what the whole of the game looks like in terms of item dependencies:

The escape through Phazon SRX88 will have a number of areas blocked off.

The only way to access SRX88's Phazon Dimension counterpart, is to restore access to your ship and fly to SRX88, then finding a portal there.

As soon as you get your Suit, you'll find a portal leading back to the "normal dimension", but you'll not be able to open the doors without Power Bombs. Furthermore, you'll still take damage in the Phazon Dimension. At this point though, you have a number of options available for you to explore, and different directions you can go. And plenty of Metroids to kill.
I love that feeling of "Okay, you have all your stuff back, and you've uncovered how this game works. Now the rest is open to you. Have fun!" in games. Even if the first half of the game so far has been fairly linear, that open feeling feels earned.

Well this is a surprise!
The Black Zero Suit is actually required. Collecting it will unlock a Save Station later on that will restore all your energy, which is the only way you can survive in this "dark world". You'll see a couple of other Metroids, but you still don't have your suit just yet, so you can't even scratch them. To paraphrase AI Adam, "If you see [them], just run."
The sequence ends once you've restored your suit in the dark version of Trozodia.

Much like in Zero Mission, there's a ZSS section. Unlike ZSS, you could run into Metroids you can't damage (and you'll just have to leave them for later). You can also find your ship, but you'll get a little cutscene saying you need at least 35 Metroids killed and your Suit restored, before you can fly it again.

During the escape sequence, it IS possible to get all of the items, upgrades, and Metroid encounters in the escape path with the sole exception of the one Missile Tank blocked by Power Bomb blocks. If any are missed, it'll be a while before the player can get back here to nab them. Including that Plasma Beam.

I've noticed that a lot of the game's design deals with funneling the player towards the required upgrades, by using pitfalls and hazards that can't be traversed back without that upgrade. Sometimes Save Rooms will be placed in those locations, which can force the player to progress in order to return, sometimes temporarily blocking them from going back to nab expansions or Energy Tanks until they can get past a boss. From what I can tell, every boss CAN be beaten without getting hit, and CAN be farmed for Missiles should you run out.

Also: ooo, new item!

And yes, you have to go into the hot areas to find the Varia Suit. Thankfully, you can have enough Energy Tanks by this point to explore the hot zones safely for a bit, enough to find where you're supposed to get the Varia Suit. This is actual training / foreshadowing for another hellrun in the future.