Yes, absolutely unpredictable. It sounds funny that it is increasing now then just a minute it will decrease. It is not stable. The roller coaster of price. It goes up and goes down.

I think it is normal to go under that circumstances. To lose and to earn.
You've lose big amount of money  it is not acceptable if it will happen again. Much better to double time your effort of investing and make your strategy more effective than before. Make everything good for you as soon as possible.

In my opinion bitcoin popularity is natural given in crypto name. No matter what happen in the next few or more years still bitcoin popularity is there it will not be demolish for it is natural characteristic of bitcoin. The features is loveable for being the first coin in digital currency.

You should by now or else regrerts will come in the future. Bright future of ethereum is clearer every year goes by. You have a big chance to grab it now or maybe later (not sure if it will goes down or maybe it goes up).
 That is my opinion.

It shouldn't be, the private key is the only key to open the wallet unless he tried to enter it in other device or in phishing site that looks like the same. Anyway, loses is lose it will never back. Better to move on and learn from what the possible mistakes why it happened.

If I have enough bitcoin in my wallet I wouldn't trade my etheruem for the sake of adding bitcoin in my wallet. Ethereum is very potential coin in crypto so it is good like bitcoin to store in the wallet. In case I have some altcoins  I will trade for ethereum sake and bitcoin too. This two coin is the first priority to earn.