Research the appropriate laws for your property and follow best practices to avoid any discrimination accusations: use the same criteria for everyone who applies, know what the protected classes are, which questions are legal to ask and which aren't, and only deny applicants for legal reasons etc.

I am not going to say there is a best lending model, but brokers legally can not be paid as much as IMB's, and I tend to be cheaper in more situation than not when comparing a loan estimate from an actual lender. 

If the legal balance is already at 60% LTV for example, would a seller expect a higher amount than the legal balance possibly?

Registered agent is in case you need to be served legal papers, they will accept them on your behalf unless you would want to be served at your home or work, and technically it must be a place where you are available between working hours. 

Is anyone aware of legal issues with this?

https://www.biggerpockets.com/forums/51-tax-legal-issues-con...

DSCR guidelines vary by lender and working with a mortgage broker or lender that specializes in these mortgages can help get better terms such as a DSCR 1 ratio vs a DSCR 1.2 ratio.

Folks will generally have a fantastic rental strategy but they don't fit the lense of regular guidelines.

Fill out both medical and legal power of attorneys.  

If you stay on the mortgage, you need to write out a legal agreement as to what happens when she sells (or if she finds another boyfriend/gets married).