Half‑bricked Phone Fixed!

This is an older (kitkat) device which I wanted to factory-reset before selling it, but alas, it wouldn't boot anymore after that. This didn't surprise me because I'd made a lot of changes to it, on a system level.
Actually, it did still boot, but got stuck loading the UI, hence half-bricked.

This is the device I'm talking about, but I think the advice offered applies to all (older) Android phones. If you experience similar problems.

So, I thought, if I can get a hold of the factory image, I can re-flash that, no problem. Acer still has official OS images on their website.
But it wouldn't install, not through the phone's recovery mode, not with fastboot.

Then I found an article that outlines a way of manually flashing the required images with fastboot - but looking into Acer's OS image zip file, there's only boot.img and none of the other mentioned .img files!

Fortunately, I found this site which stores many softwares and documents for many devices - also my E700. In the end I decided on this OS image, because it's identical to my phone's Build ID.

That image contains a boot.img, a system.img and a few more (but not all those mentioned in the article).

So I started manually flashing, only to find fastboot getting stuck at erasing 'system'... (I tried to wait it out for about 45 minutes). Some more searches told me that I'm not the only one experiencing this, and that there's no real advice/fix (just the usual voodoo stuff).

Then I found that fastboot has a -u option to not erase partitions before formatting. That did it, system.img flashed.
(Prior to that I had erased all caches (also dalvik) and data through recovery mode; something I did a lot during the events described here!)

Reboot and all good! Phone unrooted, unbricked and factory-reset.