1. Not with Eliptic Curve function.
2. Theoretically if the problem of controlling space/time is solved.
3. Use the best security methods available and don't take shortcuts to save pennies.
*facepaw*
1- Is it possible one bitcoin address has 2 or more private key case of pigeonhole principle? if yes, we have to worry?
Yes. Bitcoin addresses are RIPEMD160 hashes, so of course there are more possible private keys than addresses. But it is not a problem, because 2^160 is still an inconceivably large number.
Yes, a break is possible; but no, it will almost certainly not be a problem. MD5 for example is horribly broken, but not in any way that would make it unsuitable as a proof-of-work function.
3- This money based on technology and new technologies are on the way, is it possible to have a hacked system in future?
Well-designed cryptosystems are not broken overnight. Rather, weaknesses are gradually discovered over time. If any such weakness is discovered in Bitcoin, there will almost certainly be plenty of time to fix it or develop a new system.