Earendil's star
 

I wondered if anyone has any thoughts on why the silmaril taken back to valinor by Earendil was not broken to restore the light of the two tree's?, surely Feanor could have been released from the halls of mandos to undertake the job?

Welcome, element!

By the time the Silmaril was returned to Valinor, the time for healing the Trees was apparently long past. Yavanna said at the time of their death:

Silm Of the Flight of the Noldor

Surely the Trees were well into decay by the time of Eärendil. Though natural death was arrested by the power of the Blessed Realm, perhaps the poison of Ungoliant caused the roots to rot quickly, taking away any chance of reviving the Trees after much time had passed.

Not to mention, why the Utumno would Feanor do the Valar's bidding, taking his creation to restore the light that they lost?

Feanor wasn't supposed to be released from Mandos till the end of time for a reason, you know...

Atonement :D Perhaps, barely a few centuries after his death, Feanor was still proud as ever.

But theoretically speaking, would it take all three Silmarilli to heal the Two Trees? Would one Silmaril work, or would they work as ordinary medicine would, with all its proper doses and everything?

Since all three had identical properties, I would think the light of any single jewel would do.

I was always under the impression that in order for Yavanna to revive the Trees, Feanor had to willingly surrender the Silmaril and break it. He was the one who invented the substance of which they were made, and though the Valar might break it, their methods might have destroyed the jewel rather than open it to release the light. In this, I think it is similar to Sauron's Ring. He made it, and he might have unmade it without destroying it, but the only way in which another could release or nullify its power would be to destroy it. Moreover, I do believe that Tolkien wanted the aspect of a willing sacrifice to be part of it. When Feanor refused to make that sacrifice, no one else could do so, for they had not made the jewels and thus it was not their sacrifice to make. In any case, by the time Earendil arrived, the point was moot, as the Sun and the Moon had already been fashioned from the last fruit of the Trees and a new age of the world begun.