1) Do you agree this configuration is equally secure as a configuration in which a dedicated offline system?
I'm not etotheipi, but no this is not as secure as an offline computer. You could have a virus on your computer that gathers your wallet information while you are offline and the wallet is unlocked, and then transmits it once the computer is back online.
2) The biggest risk here (but also in the proposed configuration with the dedicated offline system) is keeping the offline wallet files physically secure. Especially as you will want to keep backups (flash storage is frail). Do you agree?
I think the scenario I outlined above is a bigger risk. At least with an offline wallet, you know that it cannot be attacked except by someone in the same physical location.
3) Why do you recommend making a paper backup? Personally I really do not trust paper (it decays even faster than any other sort of media). I would like to hear your thoughts because you have obviously put more thought into this than I.
You can make archival quality prints, which is I think mostly a function of the paper used. The reason for backing up on paper is that digital media can always fail. Paper is long lasting and easily securable.
4) Finally, atm I already use Armory (although not yet for the majority of my funds) in a configuration with 1 (hot) wallet on a system connected to the internet. I have encrypted the wallet with a very long pass-phrase (>50 characters to give an indication). Will using a offline configuration with a real and watch only wallet really improve my security? What exact use cases do I protect myself against?
If you have a virus, it can log your passphrase when you enter it, and then empty your wallet at its convenience. This is impossible with an offline computer keeping your wallet. I recommend the Raspberry Pi. It is cheap, low powered, and eminently hackable.