Welcome to Just Answer. I am here to help you resolve your tax and finance concerns. Please feel free to ask anytime you need extra help.
If you keep making lump sum payments and they cover a number of monthly payments so that you are never more than 3 months behind you will fine. A better approach would, of course, be to schedule regular monthly or quarterly payments. However, as long as this gets paid and is not allowed to be in arrears more than 3 months , no problem.
You are correct on the first item, as long as you do not get behind more than 3 months you will have no issues.
The interest should be calculated on a payment made basis just like a home mortgage/credit card. This will produce a potentially larger interest amount but since you are paying it to yourself it really will not matter and if the loan was used for business purposes may even produce a heftier tax deduction. Amortization schedules are available online for free and are very easy to use. If you want to really get the most for the interest deduction have the software do the calculation based on the actual payment dates.
Nice to hear from you again.