[Mithalwen]

It is a slightly odd concept given the nature of Elvish life. If the Valar did such a thing they would in effect be confining them to Mandos for a period until their contemplation of their wrongdoing had brought them to a state of contrition such as they could be permitted to return to life and perhaps make atonement. Presumably the Valar could effect a similar confinement without having to actually execute a death sentence.

The doom of Mandos suggests to me that death is a consequence of their actions rather than a punishment for them.

I wouldn' t like to make assumptions regarding Tolkien's attitude to the death penalty which was still used in England & Wales at the time he wrote LOTR certainly, however Gandalf's words to Frodo about Gollum were a factor settling my feelings on the matter. Certainly though the death penalty exists in Gondor - Faramir says his life will be forfeit for letting Frodo go and the "penalty of old" for Beregond's crimes but they are remitted.

It just doesn't seem a likely scenario to me.