1. What will happen to alt coins when Bitcoin becomes a legal tender? Will they have a massive growth,or would people sell and jump into Bitcoin?
Nothing in general. We're just seeing the natural growth for bitcoin and its adoption.
2. What would happen to alt coins when Bitcoin becomes relatively stable( when it is fully mined and will solve its problem of price volatility)? Will alt coins survive the test of time? Will new alt coins be created later? What would become of the various countries'currencies, especially the Dollar (the global reserve currency) after Bitcoin's general acceptance?
Creation of newer altcoins won't be stopped and that is because a developer think that he's better than the others and solving their scalable problems.
3. If alt coins survive in the next century what is the reason it would.
We will never know, we might be no longer here in this world by that time.
4. Should alt coins be traded rather than HODL?
Yeah, I guess so that most of them should be traded.