Amicus
I don't think we have enough data to predict the damage that would have been done to a metal airframe and I would suggest that it would not be the ELT battery that would do the damage but rather the burning of what it sets fire to.
As to the time the repair has taken, it looks to me as if the legal wrangling and getting the logistics sorted has taken longer than fixing the aircraft. I think Boeing have done rather well, after all this is the first major field repair of a large composite airliner being done by a company that is not yet well versed in composite repair techniques and no doubt being hindered by those who know a lot about metal aircraft and very little about composites.
I would very much like to get a look at the repair to see what techniques were used.