Sorry for the delay! There are headings for your screen reader:
Connectors
I read that you’ve documented a lot of this, but there’s a few things to be wary of when removing the mainboard:
The wifi antennas and display cables run in a vertical channel from the top of the mainboard. This channel is directly above the left display connector on the board (top edge, above the heat pipes, to the right of the wifi card). These cables will get in the way when removing the board, so try to pull them a little bit up and away from the board before taking the board out.
You will need to remove the interposer by its four screws to take the board out. It has a pull tab that stands straight up in between the two horizontal rows of screws.
Screws
I unfortunately don’t have a 16 so I can’t take measurements, but here’s a description of the locations from looking at a repair guide. Let me know if you need a better description of any of them.
There are two screws under the interposer door holding the expansion bay in place. They’re directly above two very distinct bright orange squares if you have enough vision to make that out. By feel, on either side of where the interposer sits, there will be two raised metal circles in a horizontal line. The closer of each of those circles to the interposer is the screw. It might help to put your thumbs on the interposer and rest your fingers on the chassis blocks over top of the expansion bay on either side of the interposer. Bring your fingers inwards towards the interposer and you will find the cutouts that the pin and screw on each side sit in. These two screws.
I think you mentioned the one other screw in the interposer area. It’s to the right of the interposer. You can follow the left edge of the wide right side heat pipe as it curves to the right and your finger will be directly over that screw when it touches the back wall of the chassis.
Other than those three, there are five screws directly on the mainboard. All of the screws have wider, flattened heads, and use the Torx (skinnier) side of the supplied double ended screwdriver bit.
The top left corner has one above the heat pipes. The two left side heat pipes curve towards the back of the laptop, follow their right edge until you hit the chassis wall of the expansion bay and it’s in the corner the heat pipes make with the chassis wall.
In the bottom left corner, the board isn’t an exact rectangle. The corner has a square cut out of it where a raised plastic speaker bracket sits. The mainboard screw is directly to the right of this plastic. Note that there is a screw sitting above the mainboard inside that raised plastic that feels like it’s at the exact position of the mainboard corner, but it’s not connected to the mainboard and you don’t need to remove it.
The bottom center has a screw. Find the M.2 connector for the storage SSD and run your finger from the top of it all the way to the bottom edge of the board. You’ll run into a metal standoff for a midframe screw sticking out of the board, and the mainboard screw is directly on the standoff’s left.
The bottom right screw is on the same vertical level as the bottom left. You can find it by putting a finger on the speaker connector in that corner and running your finger diagonally down and left.
The last screw (top right) is directly above the fingerprint sensor connector. This one is also close to a raised plastic part with a screw that you don’t need to remove, so don’t be fooled.
Assuming the cables are all disconnected and out of the way, you should be able to grasp the curved heat pipes near the top corners of the mainboard and tilt the board up towards you with its heat pipes attached.
