Shortly after barricading the vent, you'll find the second P-Sec Pistol Weapon Upgrade. (2:09) You should have no problem spotting it while playing.
In the storage room on the optional path, you can find the first Pulse Rifle Weapon Upgrade. (8:50) Grab it (and the Text Log, at least) quickly though, as you don't get much time to look inside the room.
This video ends shortly after opening the storage room (containing a Text Log and a Pulse Rifle Weapon Upgrade) on the first optional path of the Chapter.
This video begins shortly after opening the storage room (containing a Text Log and a Pulse Rifle Weapon Upgrade) on the first optional path of the Chapter.
If you want to conserve ammunition, use the female Slasher's explosive pods to your advantage by grabbing and launching them back with Kinesis.
The third Plasma Cutter Weapon Upgrade can be found on the platform just before the walkway/bridge that collapses. (6:48) You have plenty of opportunities to grab it from a distance.
I accidentally managed to grab the Flamethrower from a long distance, so the video doesn't make its location clear. While playing, you should easily spot it sitting on a shelf behind the left storage box just before entering the vents.
This video ends after approaching the first blockage in the vents.
This video begins after approaching the first blockage in the vents.
When Weller tells you to "make up your damn mind", it doesn't matter which direction you pick. Both lead to the same place and lack collectibles (which is a rare occurrence in this game), but I find the right path more interesting than the left path.
Contrary to the first Dead Space game, the Flamethrower is actually fairly effective against enemies in Dead Space: Extraction. If you happen to be carrying the Flamethrower, use it to your advantage on crawling enemies (such as Leapers and Swarmers) as that weapon is particularly effective against them. Using Stasis on a Necromorph in close range makes it effective on them as well.
When looking at an alcove just before a Leaper attack, you can find the first Flamethrower Weapon Upgrade. (3:58) Be quick, as you only have a brief moment to grab it before you focus on the Leaper.
This video is a great example of how the game is lenient on you even on the Impossible difficulty mode. I took a fair amount of damage throughout this Chapter and I even (stupidly) performed two sloppy puzzle failures, yet I still pulled off a solid five star rating. From my experience, it seems like Perfect Reloads and the percentage of unique items (Weapons, Logs) collected contributes most to high ratings.