Hi! I'm a mainlander.In most areas of China,people usually use Mandarin for convenience.Almost people use the simplified Chinese whenever they write or print the words.But when I was a freshman,I met a Taiwan teacher who taught us English.She used the traditional Chinese and I thought the characters were good.Also,I think the traditional Chinese should be preserved because it owns the ancient characters of China and it's culture,it's civilization.

The second one is common used.And the first one is often used in writing.

This sentence is wrong.You can say 他经验很多or他有很多经验

In terms of grammar,it's right.But Chinese people usually say 你学了几年韩语or你学韩语几年了