Using the correct SD card image? is the SD card inserted? I tested all pre soldered boards before shipping them.

If there is no continuity to GND from the 5v and 3.3v that is good but there should be something there, some resistance. I can't say without looking at it myself if the pi is dead or not or it's bad soldering. If no green LED activity is showing on the pi with a SD card with the mintyPi image on it ...

That looks surprisingly decent so how did you end up with that mess on the other board and not on this one?

I'm assuming it might be shorted to GND, check continuity on the 5V GPIO to GND and also the 3.3v GPIO to GND. Removing the pi is hard to do if you do not follow the steps in the video and have similar equipment.

I would like to see the back of the pi to see how bad that looks, if you are not getting the results like in the build video do no continue. It also looks like you are using the wrong iron tip and very very likely the wrong kind of solder for this, and too much of it.

Looks like the wrong image on there, are you using the mintyPi Lite image? and not the V3 image?the Links to the right software are on the video guide's description.

That battery reading is so wrong that you are either not using the multimeter properly or somehow you doubled the voltage output of the battery which is impossible. Other than that and you not showing us any pictures of your soldering can't suggest anything else other than the soldering could be bad...

Looks like bad soldering, you likely used flux and it's causing shorts between the pi and controller pcb. If you did not use flux it's also the soldering as random button presses occur from buttons being shorted to something else.

Do the buttons work without the 3D printed part there? just the buttons on top of the pcb? I think it's either a soldering issue as it's 95% of the time and you have have too much of a gap between the pcb and 3D print making it not have good contact with the button membranes. And why are you using t...

This is for the V3 where it's a Push button power switch not the Lite slide switch, totally different hardware.