1.) This depends. On something factual and objective, opinions should be extremely malleable. You can adhere to your subjective opinions as strongly as you wish. However, note: even subjective opinions (ethics, who you love, etc.) are based on objective premises. If those premises prove faulty, or if your reasoning for your opinion based off of those premises proves faulty, then you should reconsider your subjective opinions.

2.) Objective opinions should be based on reasons alone. Subjective opinions should be based on a combination of the two.

3.) Yes. While I am friends with people whose opinions differ radically from my own, I am often irascible around them. It's not necessary that you see things exactly my way, however I get along far better with people who do see things my way.

4.) Based on whatever they are fed first.