About the elves showing up at Helms Deep
 

Would it have been better if Rivendell and Lorien sent the elvish troops to the Battle of Pelennor Fields and the Battle of the Black Gate instead?

What, can the Elves predict the future? Or teleport over 100s of miles?

Besides, as Legolas says at Helm's Deep, no Elves would come: their own homes were under attack.

Elladan and Elrohir came, as they often did when there were opportunities to kill orcs.

Elladan and Elrohir came in response to Galadriel's summons to the Dunedain, missed Helm's deep and only knew vaguely that Aragorn was "in Rohan."

Besides, there's rather a dirrerence between a small for of 30 riding hard, and a battalion marching on foot.

I suppose it really depends on what dramatic purpose having Elves at the battles in the South serves from the point of view of the films. We can't assume that any motivation or "historical" fact from the books holds true in the "world" of the films; we can only work with what the films give us. I think the point was to show that the people of Middle-earth were not as estranged and disunited as it seemed; if that's the case then the question is whether the second or third film of the trilogy was the better place to demonstrate that.

Frankly I think some Dwarves ought to have shown up too. After all, all bets were off by that point.