From my understanding usually condos aren't so restricting as a co-op.

Ji Hun Seo I would stay away from a co-op as my first investment.

My sister is a stay-at-home mother and is open to being what it looks like most are calling a "co-host". 

By me, a co-signer just means I have someone to sue if the rental goes bad.

I have been buying and selling properties for the last three years, and I have never had a problem in getting a COE extension, some times for as long as three weeks!  

Hey Katrina, I have experience in hosting on Airbnb and would love to speak with you further about a co-hosting position.

Furthermore, by circumventing your agent, you only lose your representation, not save anything on commission as the Seller is most definitely under contract with HIS Realtor for the listing, and any sale would result in HIS Realtor collecting the entire commission, and because you cut your Realtor out, His Realtor wouldn't be required to offer a co-broke. 

One of my options was to have a co-applicant with great income to help with the DTI and qualify for the conventional loan.

Is there a way I can use a co-signer or guarantor?

If it doesnt work : get info from the neighbors, OR pay a co ie, Lexis Nexis (expensive but there are cheaper ones) to find the person, OR google the owners name and make a hundred calls to members with same last name near the county of the home or outside the area (more work).