Quick terminology fix: There is no OTG support. There's host-mode support. They are different things. Host mode support lets the N900 be the host when you tell it to, for a USB device. OTG support would let the N900 decide on the fly whether to be the host or the slave (or whatever the term is for the non-host).
Anyway, what I would do is run the power kernel uninstaller (you need to be connected to the internet, I believe), to revert to the normal kernel. Then, if you want to be safe, reboot or run "sync" as root in XTerm (though that's not necessary in my experience). Then uninstall the power kernel in the actual app manager (to get apt-get and dpkg to think it's uninstalled too). Then go and reinstall the latest power kernel from the repository. Reboot. Then run uname -a in terminal, and in the text it gives back, it should say "power45" in there.
Then if the camera stuff still isn't working after a reboot, uninstall everything that uses FCamera drivers, to make sure that those drivers are uninstalled. (Optionally reboot and then check to see if stuff like flashlight and normal camera are working again.) Then reinstall those apps again, reboot, and they SHOULD work.
I'm not sure if that's literally the ONLY technical difference, but I think that's basically what it amounts to for an end-user.
I honestly don't consider myself an expert, but that's my understanding. As for the possible next question of why the N900 can be made to support USB Host but not USB OTG, I think there's some hardware limitation in the port the N900 got. It takes a lot less technical effort for a device to just host a client device, then to communicate with the other device to determine which one should be the host and which one the client.
Edit:
Wikipedia lists three advantages of USB OTG, or rather, three different protocols, which provide some sort of advantage.
1. Devices with OTG can detect when something is plugged in, by measuring the capacitance of the port. So, if you have a cable plugged in, but it's not attached to anything, it can detect that. If a device gets plugged in, it can 'notice' that the device got plugged in. With Host Mode, the N900 never "knows" there's something plugged in. You just tell it to become host, and to mount what it detects.
2. When both devices have OTG, both devices can decide when you use power on the connection or not. So with host mode, only the N900 can decide whether or not to send power down the USB port to interact with the device. If you had OTG in both devices, the N900 and the other device would be able to tell each other when they don't need anymore power in the connection, so battery consumption is decreased.
3. OTG allows 'switching' without reconnecting between host and client roles for both devices. So if N900 had OTG, and I plugged it up to an N8, they could both write data back and forth between each other (as opposed to just the 'host' being able to do it until you reconnect or make the other one act as host), because they'd be able to tell each other to switch - so a client could communicate that it needs to become host for something, and of course this is done in moments, so for the user, it's like either device can write things / send data to/from the other one, instead of just the designated host being able to do it.
OTG support has one pin grounded upon connecting a special USB cable, which switches the device to host, that's why you can't use the same cable + adapter you use with the N900 for OTG on the N8, you need the special adapter that comes in box.
Anyways, we're going off topic.
Yes, fcamera doesn't work for two devices here on power45
