Leroy,
Welcome to fourms
There is nothing to worry and if you have any confusion, get it cleared here.
Let's take it one by one:
SOL is calculated from the debt of last activity seen in your account. Thus if you make any payment towards your debts, that only restarts the clock, sending desist letter cannot start it again.
Collectors can contact your relatives if they are unable to locate you. They can just verify your address from them. Collectors cannot disclose your financial obligations to a third person; fdcpa does not allow them to do so.
You can mention in the cease comm letter, how you would like to be contacted. You can tell them to send correspondence only, no phone calls to you and to your relatives would be entertained. Once they receive this letter, they should cease calling you.
Hope this information helps. Feel free to post your questions in forums if you have any.