Like Gymbo, I am also on V25 (stock, single boot, never rooted) and it happens to me too, though a bit more often, a few times a month, I'd say.
As for your suspicions, I think:
(1) Partly, maybe. While it seems to mostly happen when I wake my Cosmo up (usually by taking it out of its slipcase), I think it may happen randomly, but I won't notice until I wake the Cosmo up. I have had it happen while using the Cosmo (it's especially disturbing when I need to look up or write down something during a phone call). I mostly use Jota+ and Firefox, but, if at all app related, I'd suspect some system app.
(2) Unlikely. While I do have Word installed, I hardly ever use it, and if I do, it's just to very briefly glance at some Word document. Almost all my document editing on the Cosmo is plain text.
(3) Unlikely. I keep Bluetooh off, except when tranfering files, once in a while. I have briefly used external Bluetooth keyboards with the Cosmo, mostly to see if they worked (anyone wants an old Lenovo KT-1255 keyboard?), but I have seen no link between Bluetooh and this keyboard issue.
(4) I think so. I suspect it may be the Planet keyboard driver crashing and not restarting, until I reboot.
I can, however, only back that up by my general observation that the Planet software (their own and from their "industry partner") running on our devices, seems to be of lesser quality and stability than Android in general, and as it doesn't seem to matter which apps I may run, I am suspecting a Planet system app in general and their keyboard driver in particular.
Just like you, I have neither found a way to stop it from occurring nor any other remedy than rebooting. Non-root suggestions are more than welcome. I have tried simple things like enabling and disabling the on-screen keyboard or mucking about with basic keyboard settings. I haven't tried changing the layout to something else and back since I doubt it would work and, at least a few firmwares back, a reboot tended to be easier.
Finally, during boot, my keyboard can act up a bit too, dropping characters from my long-ish password or (quite often) ignoring Enter on the first try. Rather than the driver crashing at that point, I think Android just has too much going on to really be able to cater for real-time stuff, like the user.
I base that suspicion, on the phone of my spouse, a really slow (also MediaTek-based) Nokia 1, which, during boot, clearly struggles with its on screen keyboard and (previously) even its startup chime. Both the Cosmo and the Nokia 1 are using (by modern standards) slow eMMC storage, so I guess, it might be an I/O bottleneck during boot, but that shouldn't crash the keyboard once system is up and running.
