very interesting insight into the actions of the authorities here, thanks for posting! With a few of these detailed questionnaires filled out, the police will have an in-depth understanding of how the business of btc-24 worked. While I can not see any deceptive business practices, I do believe that such a business requires a banking license in Germany.

Really, "trying out mining on each one" is not substantially harder than running the hypothetical benchmark tool that you are looking for.
Don't worry, it is not rocket science -- just give it a try!

Interesting idea, but I don't think a constant stream of payment / transactions is viable with Bitcoin. You will incur large transaction fees.
This probably really needs to be implemented in a new coin.

You want people to "invest" in a fallacy?
You are aware what fallacy means, right?
http://dictionary.reference.com/browse/fallacy?s=t

Vircurex is trustworthy.

Now that part is true 

yup difficulty going down.
currently 8.30779881, i have seen >8.5 before...

Here is yesterday's announcement thread
https://bitcointalk.org/index.php?topic=203505.0

Short answer: Yes.

Longer answer: Yes, but it will hardly be profitable for you (will not cover your electricity costs). The competition with specialized hardware is just too large.
You might look into alternative cryptocurrencies (possibly YAC), where Mining on non-specialized hardware is still slightly more worthwhile.

if you have a backup of your original wallet.dat, try copying it over the wallet.dat that the newly installed client uses.

Look here for a list of exchanges trading Bitcoin and USD: http://www.bitcoincharts.com/markets/currency/USD.html

The volume column shows you that Mt. Gox is the undisputed volume leader, with almost 10 times the size of runner-ups BTC-e and Bitstamp.
Higher volume means higher liquidity (which is good) but also allows Mt. Gox to charge higher transaction fees (which is bad).

A 95% payout (assuming it is being honored) is quite low compared to e.g. Satoshi Dice's >98%.