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. 

There are no perfect options.  

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

I don't know what options to look into but if you point me in the right direction to research I will.

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. 

Good option for getting money for further investing?

The other two options require spending money that is basically going out the door to lots of fees and expenses of exiting your current house and getting into a new one.If you were just looking to move, that is perhaps a different story… If your house isn’t working for you, for instance.

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...