Can't restore OSX from recovery or new OS install drive

This all began when the laptop would no longer turn on. Confirmed a worn out power button by jumpering the 2 solder points on the motherboard to turn it on. Ordered new keyboard and replaced it (oh the fun of ~100 tiny screws). I may or may not (I honestly don't remember) have shut it down properly before disconnecting the battery. After reassembly, the keyboard seems to be working great (power button works, other keys work when putting in WiFi password, etc), but it no longer fully boots, and I'm having issues restoring OSX with any approach.

When attempting normal boot: Chime, Apple logo & progress bar. Progress bar stops at ~25%.

When attempting safe mode boot: Chime, Apple logo & progress bar. Progress bar stops at ~50%.

When attempting recovery partition boot: Chime, long pause, Apple logo & progress bar stops at ~25% (I'm assuming it doesn't see the recovery partition, so it eventually just tries a normal boot)

When holding Option during boot, boot options only list the main partition, no recovery partition.

Internet recovery: shows the globe animation, gets to ~1min, goes no further. The laptop is sitting a few feet from a high end router with 50mbps service.

Clearing PRAM has no effect on the above symptoms.

Booting to hardware diagnostics & running the extended test shows no problems.

I created an OSX Sierra installation thumb drive from a friend's Mac (2 drives, actually, results are the same from both). Boot w/option key, and it shows the install drive. Select it, and I'll get the Apple logo w/progress bar. If I've just cleared the PRAM, it will load ~50% and stop. If I have not just cleared the PRAM, it will show the Apple logo and progress bar, but show no progress.

On any of these attempts, if I just let it sit for a long time (hoping it just needs time), it will go to a black screen. It doesn't completely shut off, as I have to hold the power button for a while to hard shut down, then can turn it back on.

I'm suspecting (and fearing) either a faulty SSD or RAM (which of course is part of the motherboard), but I'm really hoping for suggestions of other things to try. I have double checked connections inside the laptop. Both fans are working.