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?

There are a ton of resources here and it sounds like you are on the right track already.

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. 

Are there circumstances where some investors might choose to use separate bank accounts and credit cards for each Series?