My English dictionary says for 'doom' also:
judgement
This is very important, because it means that doom is especially something that is laid upon you by the judgement of another. This other can of course be a
god/gods, so in that sense it could mean that doom is an unavoidable punishment, so it is always something bad. This would be the case with the 'doom of the
Noldor', which is a punishment for the Noldor's behaviour, spoken by
Mandos.
I think in general Tolkiens uses doom for things that are bad and are the result of a decision by the
Valar to punish someone for his bad deeds, or by
Morgoth (in the case of
Húrin). Or if someone just feels he is punished, even though it might not be so (as in the 'doom of
Men', which is how Men called death, whereas the
Elves called death a gift to Men).
To find a better answer we could do 3 things:
-analyse the
Ainulindale to see what is actually prophesied in it, and what isn't. I think many things that happened are not prophesied, so there is free will for Elves and Men.
-analyse all
Tolkien's work, espcially the Sil, to see where he used 'doom' and where he uses 'fate'.
-ask someone who has read the HOME and the Letters if Tolkien himself has answered it somewhere.
I have no time for lengthy analysis, so maybe someone else has???