Yes, in this day and age where they amplify everything, they don't amplify the harps. Go figure. Mahler wrote two harp parts and he wanted them doubled. Thus, there should be at least four harps on stage. That's not going to happen The celeste probably needs to be amplified as well. If not, it should be located closer to the edge of the stage.
The offstage cowbell 'thang' is also stupid. Mahler should have specified that if the offstage cowbells can't be heard, bring them onstage and forget about it. For offstage, you need a boatload of cowbells and two players, not one. The best thing is to have them on hanging straps and actually shake them. This day and age, they run sticks or mallets up into the cowbells themselves. I don't like it - it doesn't sound like cows wondering around in the fields. Instead, it sounds like a cow shaking it's head because of insects or something.
Yes, there's a lot of dumb stuff that doesn't get thought through, particularly in regards to the percussion. There are places in the finale where the snare drum should be doubled (same with M3/I, by the way). The 'muted' snare drum roll that's located where the third hammer stroke used to be- that should be played with a strong accent in order to mimic the sound of a baby hammer stroke (which is musically appropriate for that spot). And on and on it goes.
Oh, and metallic plates . . . that's another compromise that's employed because they're easy to cart about. If you don't have a set of real bells and don't want to use the holy doorbells (tubular chimes), a better sounding solution is have a set of suspended oxygen/acetylene tanks, but cut at different (and appropriate) lengths. However, they require a very sturdy stand and aren't easy to move about.