Great questions.
1. In a narrow sense, you can assume the 6.5% yield is the cost of debt for that subject company. You need to know the cost of all the other debt that your company owns in order to understand the average cost of debt for that company. In the next update to the site, we will introduce an implied cost of debt, which is the net interest expenses / total interest bearing debt.
WikiWealth differs in their approach to the cost of debt, because we take an industry average cost of debt and assume that each company will gravitate towards that optimal cost of debt over the long term. We make assumptions for 20 yr time periods. Individual measurements of cost can change over the short time, so we don't rely on them.
2. If we understand correctly, net borrowing is just the difference between what you borrowed and what you lent. We may need more information to understand your question.
Good luck. WikiWealth