I would continue to learn as much as you can, ask questions, and set a deadline that you will obtain your next investment.

I texted the Tenant to let them know & drafted a non-renewal of Lease letter to send w/in the next 2 days,to meet the April 1st deadline, so I don’t have to worry.

I would NOT recommend changing the locks if they don't adhere to your deadline.

The deadline is soon.

Send them a deadline date and keep moving forward.  

With one of them, who applied, it ended up with him asking me to prove I owned the property and going over the lease with a fine-toothed comb (after he missed a couple of deadlines to get things back to me, I moved on and denied him).I figured the facts that 1) prospective tenants see the property and can talk with the current tenants to ask about the property and me, and 2) that they have to submit an application and go through a credit and background check, and 3) that I was using Cozy to collect payments and 4) that I am available by phone, email and text whenever they want, would be enough to make people comfortable that they are not getting scammed, but it was apparently not enough for some people.Does anyone have any tips to help prospective tenants feel more comfortable dealing with you from afar and not feeling like they are getting scammed?

I would let the current tenant know that the optimum time for showings is at the beginning, you are most likely to find another person who is looking with similar deadlines, relocating from their former apartment.  

The only way I've found to get them to do anything is to provide a hard deadline for anything you want them to do.  

Your clients would need to go take a look, and write up an offer by a certain deadline potentially.

It is very important to file an extension by the March 15 or April 15 deadline, depending on whether you are a business or individual, because not filing an extension will result in a 5% per month for failure to file penalty, but it is equally important not to file a full tax return if your exchange is not completed by the applicable filing dates referenced above.