> What was the pronunciation in Old Norse for the 'sk' cluster,

Probably just [sk] but perhaps with a palatal before fronted
vowels [sc]. Never as a single fricative as in English 'ship'
nor as an affricate like in English 'chip'. The word 'skip'
was pronounced [scip] - i.e. closer to English 'skip' than
'ship'.

> and the rune symbols 'thorn' and 'eth?

Unvoiced and voiced dental fricatives respectively.

> Does anyone know haow vowel lengh was marked in the runic alphabet?

Usually not at all.

Kveðja,
Haukur