You are correct on both parts. I was looking at 18% on my spreadsheet, should have been 16%. Also, The interest earned on subsequent tax is the same as that which was awarded when the original tax lien certificate was purchased. I must have confused another county in that regard as well.
This is actually pretty disconcerting to me because when I bought the subsequent taxes the first time I had confirmed it the other way. Here's Yavapai's Tax Law on subs:
42-18121. Payment of subsequent taxes by certificate holder; separate certificate of purchase by assignment; fee
A. On or after June 1, if a person who holds a certificate of purchase desires to pay subsequent taxes, accrued interest and related fees due on the property, the person shall exhibit the certificate or receipt of registered certificate to the county treasurer. The treasurer shall enter the amount of the payment on the certificate and on the record of tax lien sales. The amount of subsequent taxes bears interest at the rate stated in the certificate of purchase from the first day of the month following the purchase of the subsequent tax lien. The county treasurer shall collect a fee of five dollars from the holder of the certificate for making the entries.
B. Beginning in calendar year 2016, on or after June 1 in a county with a population of more than three million persons if no prior year certificates of purchase are outstanding, if a person who holds a certificate of purchase for tax year 2014 and afterwards desires to pay subsequent taxes, the county treasurer shall issue a separate certificate of purchase by assignment for each subsequent year's taxes, accrued interest and related fees due on the property. The principal amount of each subsequent year's certificate of purchase bears interest at the rate stated in the original certificate of purchase exhibited by the person from the first day of the month following the purchase of the subsequent certificate. The treasurer shall collect a fee of five dollars for assignment of a certificate of purchase for each subsequent year. Except as provided by section 42-18201, subsection B, subsequent certificates of purchase by assignment carry the same foreclosure right as the original certificate of purchase, and partial payments shall be accepted by the treasurer as prescribed by section 42-18056, subsection C, by any party with a vested interest in the property.