I think you've got a wide range of photography covered with the assortment of lenses you're bringing. If you're strictly doing landscapes and looking to rent a wide angle I would go with a 17-40 f/4 or a 14mm prime over a 16-35 f/2.8, from what I remember during my research the 16-35 f/2.8 is a great event photography wide angle because of its fast optics but overall it was not as sharp as the 17-40 f/4.
1.) Between the 17-40 f/4 and the 14 prime. I'd still go with the 17-40 because it'll give you range and versatility over the 14 prime. and I don't think losing 3mm of wide angle on a full frame is much. Primes are sharper but both lenses are L lenses so I don't think sharpness will be a concern.
2.) I don't think you'll need a flash. A flash is not powerful enough to illuminate a grand expanse like the grand canyon. It won't even come close. A better and cheaper investment for landscapes would be a trigger release. You can set your camera to Bulb and capture the stars traveling across the night sky in the background with the grand canyon in the foreground. How beautiful would that be right?
3.) Oh whoops star trails...I mentioned that in #2.
But you'll need a trigger/cable release and a tripod. I would invest in at least a Giottos or Enduro tripod if not better. Something that can handle a lot of weight. I've got a Giottos and a Gitzo Traveler myself.
4.) I've been to the Grand Canyon but this question is too difficult for me to answer. I'm not an expert on that area. I did see tours by mule/horse and I think that might be more creative and give you a more unique look than just the top down view most people do.
5.) HDR all the way. This seems like a trip where you'll have a lot of time to set up for your shot in which case you have time to do a proper HDR. Forget bracket shooting. Do a proper HDR. I'm talking 5-8 photos depending on the time of day and range you want to capture.
ND and Grad ND shooting is for run and gun shooting where you don't have time to set up. I did that in Spain and New Zealand...mixed feelings about it. HDR will always give you a better look. But please please don't over do the contrast. I HATE it when people make their skys look like some sort of electrical storm or apocalyptic explosion is happening. I think the best HDRs are the ones that have slightly more range than the human eye captures at that moment. Just my opinion though.
Here are a few of my own HDRshttp://www.flickr.com/photos/halogenlab/8184226516/#sizes/l/in/photostream/http://www.flickr.com/photos/halogenlab/3703942284/#sizes/o/in/set-72157621035695209/http://www.flickr.com/photos/halogenlab/3703934582/#sizes/o/in/set-72157621035695209/
- Minh Nguyen