I missed U-air so I'll be adding a little on that first. I really like using it if I can force my opponent to land on a platform (and I'm underneath it). It seems to linger for a really, really short amount of time, so I use it to hit them as they air dodge into the platform (and before they can block).
As for Dsmash, I love that move. I use it a lot against faster characters. One of the few things you can do to protect yourself against fast characters. Like it was stated earlier though, power shields make it's lag quite noticeable and not particularly safe.
DI'ing down into the stage and teching isn't a viable strategy of surviving the Zelda's Dsmash IMO. If Dsmashed near the edge of the stage, you won't have time for your character to bounce off of the ground (therefore you'll just screw yourself over by DI'ing down). If Dsmashed across the stage, you could DI into the stage and tech, but unless you're at an obscenely high percentage, you probably could've survived the Dsmash by DI'ing correctly anyway (since Dsmash only has decent knockback). So yeah, unless you have a decent amount of stage between yourself and the edge and the Dsmash will somehow still kill you, I wouldn't DI down and try to tech (the tethers are a different story though, it may be worthwhile because it might be possible to successfully do it closer to the edge of the stage).