I have a few options.

They say they will sign my lease and also sent me an addendum to sign allowing them to sublet.There are a few reasons why i like this option for this property, but im wondering how to background check the company and the individual they are assigning to ensure ill be paid monthly as they claim. 

You may have to pay some costs for building out of your pocket. 

I say RE hold less than 5 years to look at other options.  

Which option is the best route business-wise is a complex decision.

I know a few deep pocket investors this might be right for. 

I've heard of "blanket mortgages" which I thought might be a better option for me so I could use the equity in both properties and only need to get one mortgage.

Either of those would be much better option then a blanket loan to (as mentioned above) the issue with partial release. 

I think other options can get you places much faster. 

Call your banks and ask them if they offer any options and try to work with them.Another option is to sell one of your property, consolidate and move on...