So my credit would be free from any mortgage (although I am on the deed).

As still being 19 my credit is still establishing but making sure I pay everything on time and taking care of it.

Can any lenders out there educate me on whether there is such a thing as 5% down, low credit score (low 600's) jumbo loans?  

There are no shortcuts to building credit, it takes time.

More info in the link below.https://www.biggerpockets.com/forums/521/topics/605679-september-2018-christyna-romero-credit-specialist-ceo-of-crt

We both have full time jobs, our credit scores are very good, and we have some savings.      

(See American Housing Survey Reveals Rise in Doubled-Up Households During Recession and For Many Families, Bad Times Require 'Doubling Up'.)And there was also a fear about jobs/income and credit.

3) Is there a usual or standard amount/percentage for the Option Fee, rent credits, and any other fees/charges?

If you haven't already, it would likely be worth your time to start having conversations with some of your local bankers / credit unions about your options around conventional financing vs.

SO what I guess I am saying is watch your DTI....Decide whether you want to have that line of credit available or if not.