A couple questions on tax lien investments

[Tim Ball]

Lets say the interest on a tax lien you purchase is 12%. That 12% is annual interest rate.  Lets say the lien you bought was $10k. If the homeowner pays the tax lien off in 3 months, do you still make 12% on your money or do you only make 3% since it was paid off before a full year?

Second question,  if you buy a tax lien and the investor does not pay it off and you decide to start a foreclosure on the property, do you run the risk of other liens on the property such as mechanic liens, IRS liens, etc.  If those liens are present during foreclusure, will you be responsible at that point for those liens now after taking possession?

Thank you for your responses.