yups, 2018 predict that there will be a year of altcoins, some altcoin will have a good future and in 2018 I think the price will go up and and give big advantage. I believe crypto will change the world in near future so we are now at begining of a new era .
i think we can not say its legal or not, because it is depend on the country where we stay. some country allow and some not allow, it depends on the rule of government from the country
I think this situation is normal. Because the trading will be in high and low prices. So if the prices is down, it is a good time to buy the coin. Then sell it when the price is high.
it depends on types of trading that you will choose. if short term trading so you should put your coin in exchange. yet, you can save it in your wallet if you do long term trading. holding the keys is not enough.