"But when Mandos was approached [by Finwe, with his offer to stay in Miriel's place], he said to Finwe: 'It is well that thou desirest not to return, for this I should have forbidden, until the present griefs are long passed.  But it is better still that thou hast made this offer, to deprive thyself, of thy free will, and out of pity for another.  This is a counsel of healing, out of which good may grow.'" 

Morgoth's Ring, page 249.  It certainly seems as if Mandos approves of Finwe's choice.

Also, even though Miriel rejoined Vaire, that is not the same as being with her in spirit only, and I doubt that Miriel could have become Vaire's chief handmaid, or could have woven her webs, in spirit form.

“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language.  (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)