try swapping a know good front cam and back cam into the device, if that works then replace the old back cam and see if it still works and vice versa. had a few give me crap from time to time but maybe that helps.
i wouldn’t rely on just coconut battery. ive had batteries where they read 100% but when you put them top real world tests they ended up being closer to 75ish%. swap the battery and let it sit on charge for a few. also try a few of those other tips. background apps kill batteries and so do location services,
that would be at the discretion of the particular “genus” you deal with. honestly i would say go to a best buy since they are a ASP and have them do it. ive had them do a few that were defective and just did the back glass myself.
could always be a defective screen. id say if you are getting parts from anyone besides a trusted parts distro center (fixit, mobile sentrix, injured gadgets just to name a few) you are going to run into some rough parts. always go as close to oem as possible as well.
i would try to boot it into safe mode. press and hold the power button until you hear two chimes from the system. if it displays in safe mode you are home free with a new drive. if not then you are looking at a hdmi port reflow or replacement, hate to say they are quite common on most ps4 systems due to the overall weak mounting points on the port its self. if you are handy with a soldering iron you can fix it for maybe $4usd if not free if you dont need a new port. if not it may vary on where you live.
so it sounds like you have a bad hard drive. i would swap in a known good drive into the system to test it out. note that any data on said drive will be gone. if that doesn’t work ill be keeping a eye on this to see how your doing.
looks like you might be able to run jumpers from the traces to get the button to work. Now top actually mount the button to the board you might have to get creative. when in doubt hot glue might come in handy if its something for yourself. if its for a customer always disclose its going to be weak but be gentle.