If this is done, then 75% of the gross rent for your current primary can be used to offset the mortgage debt of that property to 0.

Should the seller send me a 1098 mortgage interest statement?

Keep in mind that most residential mortgages are conforming loans that are resold on the secondary market and bundled into mortgage backed securities.

I am currently 2months behind on my mortgage.

However, if you need more flexible terms then look for a local mortgage broker who has access to a lot of companies offering HELOCs with most having more flexible terms than local credit unions. 

She could have chosen a much cheaper solution, like go to a coffee shop.

Both of our names on title and mortgage in my name only. 

just to save on my own mortgage How did you find this deal and how did you negotiate it?

Quote from @Jeff Chisum: If a DSCR originator (keep in mind you don’t need a mortgage license to originate) tells you the loan will not be reported on your personal credit but require you to be a personal guarantor your next request should be; “I want to see in writing it will not be reported on my personal credit”.  

So if you can collect rents into your business either by deeding the properties to your LLC (be careful of the due on sale clause on your mortgages) or by signing a master lease between you and your LLC and having the LLC sublet then that may help.