"Is there a way to renegotiate after the initial contract that does not involve terminating?"
Unfortunately no. Your ability to walk away from a deal is always the best negotiating tactic. But that's fine, RE investor routinely have to try for a great many deals before the right one goes all the way through.
"Since both sellers had provided misinformation about the rent, I felt the letter asking for rent verification would be useful."
If you are buying an investment property with an existing tenant in it, you should absolutely get a copy of the lease. I ask for these before even writing an offer. If it's empty, and you just want to figure out how much rent it will make... there are a variety of tools to help you estimate rent: Zillow rent estimate, rentometer.com, Rent Range, etc. You can try asking a local property manager, but if you're not planning on using them to manage the property they'll probably not be eager to help for free.
All the best!
- Sam