@Eric Piercey nice job on taking steps to do your first flip. Here are some thoughts about your questions. First some general observations from my experience flipping houses:
1) It is usually a 9 month journey - It is very difficult to finish a flip if major renovation are needed in under 9-months from purchase to closing wire. A lot of people say it can be done, but there are lot of things that get in the way that you cannot control. For example, once you find a buyer it takes as much as 60 days from the point the AOS is signed to get to the closing table with an approved buyer then as much as a day to get the wire in your account. Also, if you need to get permits, the inspection process can delay you as much as a month and in some cities multiple months. All listed above does not even include the actual rehab part, which has its own delays.
2) Taking on a lot of leverage is risk - No matter if you are going to get a HML or work with a company to get you credit. If you are not putting a significant portion of cash down into the deal you are by default leveraged with debt. The risks are that if things go wrong and you don't have reserve capital you could either have bill collectors going after you or the HML company foreclosing on you before you finish the project and all of your money gets tied into the house. This is all the most pessimistic possible view, but be aware that leverage and debt have risks. The benefit of using unsecured credit like WCAP Financial offers to help you get is that they cannot foreclose on your property, a HML can.
Ok now to your specific questions.
1) Are my interest calculations correct based on reuse? - No. 9% per year is 9% per year. You can play tricks on the numbers to say x% per project, but if you are paying 9% in a year then that is what you are paying. However, I do agree the more projects you do per year then you are paying less percent per project.
2) How accessible is the credit line? - using a company like WCAP Financial, you will be getting access to personal and business credit cards and lines of credit. It is as accessible as the credit cards already in your wallet. There are ways to do cash advances or pull cash out, but they can be complex and you should defer to an expert to help you navigate through the process or you could be faced with big cash advance fees. Lines of Credit are a bit different since you can write checks on them, but the interest starts immediately, so you don't get the benefit of 0%.
3) Are my numbers realistic? - Its hard to say in abstract, best to look at a real deal and run real numbers. Also, keep in mind HML is not just 12% - 15% it also includes points and fees that can be as much as another 3% - 5%. Last, keep in mind the 0% credit card offers only last for a specific period of time.
If you have any additional questions, feel free to send me a DM and I am happy to talk through more of it with you.