I visited Iceland last year with my 7D and took only 17-55IS 2.8 and 70-200 f4 L IS.
I could get as close to puffins as 2 meters. Just go to an area that allows that, like the Guided tour at Ingolshofdi.
Instead of a large collection of primes, why not take versatile zoom like the 24-105 instead?
I had 2.5 weeks of great weather, but if its rain and wind as is not uncommon, I wouldn't wanna be changing primes all the time when hiking. Just put he 70-200 f4 on the crop body and enjoy the holiday...
Thansk for the advice, I take it you did not miss at all any more reach than 200mm (with crop body). But was you 17mm wide enough in crop for the icelandic landscapes? you see, I'm leaning towards option 1 in my original post but my big fear is loosing the wide angle capability in case the 5DIII fails and I have to rely on the APS-C only.
Btw, i'm not taking all the primes, probably the 21, 35 and 85 only. The 4th lens will be either the 70-200 F4 or the 70-200 2.8 II or the 100-400, base don your recommendations ;=)
On wide angle, there where no subjects that wouldn't fit into 28 mm full frame equivalent, composition wise you may prefer wider of course, like your 21mm.
If your really afraid of camera failure, yes I would take a second full frame. Never had a camera die on me, but twice while on holiday In the USA my standard zoom died....
Really you can always take along as much gear as you want, question is if its a problem lugging around all day.....
In that regard the 70-200f4 is a lot lighter than the 2.8 or 100-400.
Some non photography related advice, consider camping ( rented an RV), saves a lot of travel and some campgrounds are amazing, like the one at Godafoss. Due to the extreme lengthy daytime, you can do a lot of photography in the evening without bothering to drive to hotels/guesthouses, that may not be near in some cases.
Also, every town has a warm pool open till 9 PM, that's almost for free, so we never bothered to use the camping showers.