Never shot the Grand Canyon but landscapes quite often.
So my thoughts by the numbers;
1, The 14mm is wide, very wide. The diffrence between it and the 16-35mm at 16 is very noticable. I would be renting and taking one of the following if I were going. 24mm f/1.4L, 24mm TSE or the Zeiss 21mm. If I could afford to rent one more I would rent the 14mm or the 16-35mm but only for the very widest shots. But I probably wouldn't even take the 14mm or 16mm, In stead of super wide I would plan on doing Pano's for the areas I couldn't cover.
5, HDR used lightly is not noticable. However it is best if you have a good set of Grad ND's. I am assuming you are talking 4x6. You can hand hold or manipulate the Grad's in odd positions to darken the bright sky. It doesn't have to be just horizontal. I have used mine at odd angles when taking pictures of mountain sides and such. You can be creative with the grad filters and stack to get what you need. I think they are a must have, as well as the CPL. But the best option isn't filters. The best option for landscape is always timing, unfortunatly on vacation we often do not have that luxury.
Yes on 4x6 (for the ND grads, the big stopper is 4X4 as it is frame filling). Still don't know to use ND Grads with a CPL at the same time without filter stacking weirdness, rotating the ND without rotating the CPL, etc. So I typically use ND grads or CPL, never both simultaneously.
I'm with you on rotating the grads to counter the sky, but with hard grads, I'm not sure how to use them against uneven (and bright) skylines. I'll darken the tops of some rock peaks, right? I thought uneven skyline + limited movement threat (leaves moving in the wide, walking people in the frame, etc.) would mean to opt for HDR instead of ND grads, but I will try both for sure.
The big stopper, on the other hand, seems fully in play for this trip. Besides water long exposure work, it could be killer for stretching out a cloudy sky. Just got it, and I am fired up to try it.
I looked at the Zeiss 21, but I won't have a
Not sure what you mean on stacking weirdness. I often use CPL with the 4x6 hand held, since I dont have a CPL that will mount in the Lee holder I put the CPL on and it just takes a bit of a steady hand.
For the senario you are describing I think soft grads would be real helpfull on the trip.
I wouldn't opt for HDR over any combination I can create with the camera and filters. HDR doesn't have to end up looking like the cartoonish look some people do. Here is the deal though, do your best with the filters and then bracket a set with the filters on. That way later if you don't feel you got the effect or look with just the filters and a single picture you can always do a bit of HDR with the set. Set up takes the longest, taking an extra exposure or two takes no time.