In regards to your question #2
2 - With 68lbs. of thrust and a 50lb design... is there a risk of going trans or super-sonic?

With your calculations of 7.833 gal of onboard kerosene which weighs 6.8421 lbs/gal, your fuel load would measure 53.5941693 lbs which means your air frame would weigh a minus -3.59 lbs, rounded off. So the answer to your question would be no.

RH