Dropped iPhone won't turn back on, processor heating on charge.

So some time ago my brother dropped his iPhone 5C. The screen stopped working, though notifications would buzz, and the vibrator would activate when flipping the mute switch, so the phone was still on, but it heated up a bit, so he turned it off (holding power button, guessed where to swipe).

He bought a replacement phone, but I just got a kit with a pentalobe driver, so I thought I'd have a gander inside, and I couldn't see anything wrong with it, beyond it lacking a contact cap under the mainboard. I couldn't find it anywhere, so I assume it was missing to begin with (may have been a refurb unit, can't remember. There was smudging on the back of the display unit, so looked like somone had been in there before me).

I tried booting (after giving the good ol' hard reset a spin) without the battery (while plugged in to power, of course), the display and the rear-facing camera (one at a time, mostly), but couldn't get anything out of it. I put a multimeter across the home button spring terminals, and got a voltage, so the phone (or at least some of it) is getting power distributed, but it refused to boot up regardless, as far as I could determine.

My power bank (which I'd plugged it into) was also observing a draw from the phone.

After leaving it on the charger for a while, the processing unit and ports (far left here) on the mainboard had heated up noticably, the processor more so than the ports.

Is this likely to be repairable at all? The only other thing I can think of is a solder joint[s] having cracked and losing contact, unless it's the actual silicone inside the chip, or something worse, like the board itself. Would the solder be reflowable at all?

Various extra pictures (apologies for the potato lighting)