Question is when you binding a spirit who was a Hermetic Magus/Maga and had a score in Arts also, can you use the Affinity w/ Ability option in binding? It says "Ability" but I think it is bc the RoP:I is not centered around Hermetic Magi and based on the spirit of the rules presented in the book it is possible to use a binded spirit as a giude to receive Affinity w/ Arts if the spirits score is not lower than the Arts score you want to achive.
Other question is do you receive a basic understanding of the spirits abilities and knowledge when you bind a spirit to yourself? Can you "use' binding as an option to measure the capabilities and abilities of a spirit?
Typically such a spirit wouldn't have any Arts anymore, unless through one of the Mysteries. It is possible for such a spirit to have the Gifted Quality, though rarely. Usually ghost would have Powers reflecting the magic from life before.
Based on the book Calebais: tbC I believed that Hermetic Magi's spirit can keep their scores of Arts - or at least the ones were important during their lives. Maybe reduced equated to their Might Score.
What do you think: can they keep their arcane abilities or is that changes to Powers as well?
They can, rarely. Usually they're converted. Look in HP and LotN for examples of what typically happens to the Arts of magi who become ghosts or who transform.