There are a few dozen houses with my target number of beds/baths/sqft that have been listed for rent on Zillow for 2+ months with 0 applicants.Is this normal or is it a sign that's there's not enough demand for rentals?

Not sure what state you are, but here your lease wouldn't be "void"  Anything that conflicts with the code would be (landlord-tenant code supersedes leases in DE anyway)I would just say you had them sign an older version of your lease in error and need them on the current one.

Send each tenant a letter letting them know you need them to sign a new lease that will contain a rent increase schedule.  

So to expand on the question...hypothetically...you have a 5/3 for rent...your tenants consist of 2 adults and 2 children....they sign the lease agreement...THEN...move in 2 (or more) adults...can you have verbiage in the lease agreement that the rent would then increase a set amount if other occupants (adults) move in...providing they pass the credit/background checks...

I recommend connecting with @Andrew Freed who runs the Worcester meet-up. https://www.candorealty.com/simplified-rei-meetups  here is the sign-up page.

It has been a couple days and have not seen an answer.If you own a rental property and you have a signed lease at the minimum of 1+ year, does that rental property mortgage (traditional investment loan) still count against your DTI?

Does that sound like a guy who knew what he was signing?

There are a few things you could offer to make the deal enticing to the seller, such as a balloon payment, but I would keep it to less than 10 years unless it is too good of a deal.You need a purchase and sale agreement with the terms and after under contract you will work with the title company to get a mortgage and promissory note drafted by an attorney to be signed at closing. 

Alongside this 5,000-square-foot Bird Streets home, buyers have the option to acquire three life-size Spider-Man figures signed by Lee.

Fragility is a sign of a corrupted mindset, one that is fighting against something.