The Chance is 1/30000 everytime u get an encounter. Chance is better, when you do Hordes. 5xHorde basically meand 1/6000, since u you see 5 at a time. A Hunt can take a Single Time, but could also endup in ridiculously high (I have seen Hunt with 200k Encounters).
As long as you breed a female shiny ot with a non ot male it stays your OT. If you have a male Shiny, it just keeps your OT, if you breed with a Male Shiny ?