Duolingo has accepted my answer "Ciamar a tha thu, a sheanair?" So is saying "t(h)u" to one's father actually in use among native speakers, or it has been accepted only for the sake of the course?
It varies. Typically, 'sibh' would be used, but it very much depends on individuals themselves. Some might use 'thu'. :)
Also, this is grandfather, although the same idea applies.
why is "seanair" lenited here?
Vocative case, "a + lenition". 'O, Grandfather!'