What happens if I find an old chest with a 100kg in gold/diamonds/unobtainium? I think this situation is quite a good analogy to your question, the consequences of which are readily known.
I am pretty sure if someone found a stash of gold worth $400.000.000.000 that WOULD actually effect gold prices. Also: Gold has 'real life' uses (production of components etc) and real world costs etc. attached to handling (weight, transport costs etc). None of which BTC has.
Yes, it would. Spain essentially demonstrated this in the 16th Century, with the New World as the "stash".