It seems you don't have a smallest clue about how bad that can be turned out.
-snip-
That was what i meant. It will definitely kill bitcoin. Nobody will trust it again.
False. Anyone who believes that zero confirmation transactions are safe is either very misinformed, ignorant or delusional (or other?). Regardless of the situation with the network, accepting such transactions always carries a certain risk. I'm pretty sure merchants do not evaluate the TX priority. When dealing with non trivial amounts, anonymous people and such you should always wait for at least a singular confirmation. For example, trying to cheat Steam like this is ludicrous as they know who you are and can completely block your account for fraud.
Note: There is no *fix* to this. Even if the block size was 100 times larger, the network could still be flooded (regardless of whether they are genuine transactions, academic research or just malicious spam).