You need this kind of hand for the simultaneous bottom corner tapping...
...while holding the power button with the other hand. Oh well.
Basically everything has to happen simultaneous - device powering on after being off, power button getting held down, and both bottom corners getting tippy taps.
Other devices with more buttons have it easier you just have to hold the powerbutton and the pageflip button, or light button. The touchscreen is not involved anymore...
Alternatively see if you can obtain the card and mount the rootfs (ext4 filesystem) and extract Kobo's regular firmwareupdate KoboRoot.tgz over it. That would fix the issue if it was due to a mod damaging any of the original firmware files.
For restoring the full sdcard image, I'd use a new microsd card and leave the original one intact, as that one also stores your serial number and such things. It might still come in useful. Or otherwise produce a backup image of the original in case you need it or anything that was stored on it, before overwriting it.
I think I had a video uploaded of how to perform the factor reset on the H2O but the video site I uploaded it to does not exist any more.