Is it worth to invest somewhere I will be leaving for another state?

Maybe tell them to leave while Construction and offer them first right of refusal to live there again after construction is complete?

Or just put "$1000 - unpaid rent." and leave it at that?

So if I spend 30k, then I sell the house for 200k it would pay off the primary note for 100k, then the HELOC for 30k, and leave me with 70k to put down on the next property?

After seeing what needed to be done I was ready to leave and a tenant walked out let's call him "D". 

Leaving you to wonder, "why can't I seem to get any deals??"

I believe, based on state law, we could demand they cover rent through the end of the lease (another 9 months) but my suspicion is that if they decide they're leaving, they're going to leave and the only leverage we really have is the deposit (one-month's rent).

You might have to leave a message the first time because I might think you are a sales call and I usually will not answer because of it. 

Id dump it in the open market .we are  at the top so make hay while the sun is shining ! 

That's a term used meaning to locate people who moved away, leaving no address, and you have to lfind them.