unable to restart in recovery mode

Reset the NVRAM.

You have to disable the firmware password (or provide it when prompted) before you can start up in alternative modes, including safe mode, Recovery mode, or any other mode besides a normal boot. Try disabling this first and then see if the system will load into Recovery mode.

I cannot make a safe-reboot due to FileVault2

That's right.

I cannot reset the NVRAM

Yes you can, and I think you already did. If you tried again to get into Recovery mode, and failed, then something is wrong with your installation. That's also indicated by the error you reported in your original message. The pre-boot authentication dialog isn't working.

You need to deactivate FileVault. Do that either by booting from another volume or by starting in target disk mode and connecting the iMac to another Mac running OS X 10.7 or later. In either case, launch Disk Utility and decrypt the boot volume (File > Turn Off Encryption).

How to use and troubleshoot FireWire target disk mode

I'm assuming you have backups. If you don't, you need to back up before you do anything else.

• I cannot make a safe-reboot due to FileVault2
• I cannot make a reboot on the recovery partition (the subject of my original question)
• I cannot reset the NVRAM

You are correct. FileVault will prevent booting to Safe Mode by holding the Shift key at startup. You also cannot reset the NVRAM (PRAM) with the firmware password set. These features will have to be disabled in order to perform these actions.

Booting to Recovery should be allowed if you supply the password, but if not then there is a problem with the recovery partition. Usually reinstalling OS X is a simple way to set up this partition again without harming your user data, applications, and settings. Can you use Apple's Recovery Disk Assistant program (http://support.apple.com/kb/DL1433) to create a bootable thumb drive from your Recovery HD? If so then you can use this to reinstall OS X just as you would the Recovery HD itself.

Alternatively you can hold Option-Command-R at bootup to invoke the Internet Recovery mode if your Mac supports it. This will have your system download the recovery tools instead of using the ones installed on the Recovery HD partition.

One thing you always have to keep in mind about Apple Support Communities is that it's not an Apple Support portal. It's a discussion forum. The content is provided by end users and is worth what you paid for it. None of the technical information given out here comes from Apple representatives, and none of it should be considered definitive. Some of it is helpful, and some is wrong. The difference may not be obvious. The fact that someone offers an opinion doesn't mean that he knows what he's talking about. Use caution, do your own research, and make backups before acting on any advice you're given here.

Perhaps for completeness whenever you post this disclaimer of yours you should also include a link to Apple's Terms of Use which basically outlines this and more: https://discussions.apple.com/static/apple/tutorial/tou.html

It does appear that this may be the case. The Intenet Recovery option may also not work, as it requires a relatively newer system (2010 or later, I believe), whereas from your profile above it appears your system is from 2008.