This idea of pump and dump, and price manipulation is just plain wrong.
The manipulator is the one that loses, no one else.
Look at the recent Mt.Gox hack to see what happens when someone "dumps" 500k of BTC. The loser was the owner of the 500k of BTC, not the people who snapped them up for ten cents each.
In your hypothetical: firstly, if Bitcoin were big enough to be used by multiple countries, the scope for dumping would be severely limited. Secondly, the dumper would drive the price down, getting less for their BTC than they were originally worth. Perhaps they are willing to take that on the chin, but it is them that has lost. If they have lost, then others have won. Finally, if Bitcoin is big enough that a country is using it as their currency, there is no problem with them continuing to do so. If the size of the economy merited $X pre-dump; then post-dump the price will recover.
You forgot to calculate in the fact that once people start seeing the btc price drop 10-20% in one day all the idiots are going to be, "OMFG, THE WORLD IS ENDING SELL ALL YOUR BITCOINS BEFORE THEY ARE WORTH NOTHING." and drop the market another 20-30%.
Greed and fear is what drives the market and those two emotions lead to speculative booms and busts, always.
We all saw that happening when btc went from 30 to 10$.