"Should you" as in "is it morally/ethically wrong?" No, it's not. "Should you" as in "is it a good idea?" No, it's a terrible idea. And I think you know that on some level.

I don't believe in the idea of "thought crimes" so morally you haven't done anything wrong. But its probably unhealthy to do that.

No, nothing helpful will come from nurturing that fantasy.

A relevant proverb I heard somewhere on a similar topic: You can't prevent a bird from flying over year head. But you absolutely can prevent it from building a nest in your hair.

No, it is not healthy and will go nowhere or could turn into an obsession in the worst case scenario. It is better to move on and find someone who is interested

Don't do that to yourself. You need to move on.

No, once you know it's not going to happen you need to do what you can to let the idea go.

Holding into it is just tying an anchor to your happiness that will only get heavier.

As a pure act, I can't see that it causes any harm.

However, it will very likely lead to frustration, anger, and poor treatment of the person in question. Those things are bad.

In what reality would that be a healthy thing to do?

No. I know it hurts, but you need to let go.

No, it will just hurt you in the end.

As always, the dose makes the poison. An occasional fantasy won’t harm you, but an obsession is unhealthy and will make your life hell.