Glorfindel, I have to tell you that I think that you are rather a genius. Brilliant in fact!  I really can't think of a downside to this.  Everything I have been able to dream up has a potential butterfly effect, shall we say?  The ending could be more dark than it already is.

That's true Glorfindel, and good quote from Star Trek.

I think that for almost any appearance of a Time Traveler, unknown variants of the future would occur. Perhaps if you indeed took the two Silmarills from Maglor and Maedhros they would have attempted to take them back from you and therefore start a new Kinslaying.  Perhaps an Orc band come at unawares and taking the Silmarils deep underground, give them to a Balrog....  And on and on.  Time travel is certainly a brain twister.  Just like in Star Trek, entire worlds can disappear from our current time, without us even knowing they were ever there.

Khazad  at the peak of its glory and a second trip in its time of dread just before the fellowship passed through!