150W is a lot more power than what’s in the box could consume. It would be interesting to know what what power delivery profile the USB ports conform to. Apple, of course, does not say.
Is the NXP 80V18 PN80V really an NFC controller: iPad doesn’t do NFC. Could it be a gyroscope or something?
Might that beefy bracket be made of Liquidmetal (see https://en.wikipedia.org/wiki/Liquidmeta...)? That is, is it surpisingly stiff? Apple has an exclusive license to use Liquidmetal in consumer electronics, but seems not to have used it extensively.
And you can see where the connector would be soldered at the bottom left of the board.
This procedure works to add a SATA hard drive/SSD to a PCIe SSD-only Mac Mini using iFixit's Mac Mini Dual Drive Kit. The one thing that's missing from the kit is SATA cable connector bracket (and screw). This is not the end of the world: the cable is a snug fit in the logic board socket, so the bracket is only a safety measure.
What's under the can in the top right of the third pic? My guess is that it's Apple's "secret sauce" timing controller.
I second that call for further investigation of the charger. If you plug one into a Mac and investigate it, first with System Information and then with USB Prober (the latter being an optional download once you have XCode), you find it's got a lot of USB personality. It claims to implement the HID (Human Interface Device) profile with a transfer speed of 12Mb/sec, and provides more (uninterpretable) information when attached to a Watch than when it is not. So there's definitely smarts in there. To what end? Beats me.
I'm kind of surpised that the Bluetooth chip is v3.0: Apple's specs say the Magic Trackpad is compatible only with Macs supporting v4.0.
So how does the Bluetooth antenna work? There seems to be an antenna connector on the logic board, which does not obviously have an antenna etched in, but none of the steps shows anything attached to that connector.
Looking at several of the high-resolution photos, my best guess about the two mysterious spring contacts is that they sense when the back is off the device.