I think the Circular Polarizer filter and Graduation Neutral Density filter is most suitable for the hot Arizona sun after gone through all your postings!
on the Grad ND will have a three different type of density 2, 4, 8 for example! will use it according to the lighting condition!
I am using ND8 myself when the exposure is too strong! may be you can try it out with your little friend in your local camera store!
as for the CPL, will saturate the colouration more also to brings out better on the clouds formation and subject! so that will have more soft and gentle lighting and colour!
the UV is something to protect the camera body lens to prevent scratches!
Hope this might help
p/s you can add cpl and ND on top of each other also other filters, just check and prevent vignetting on the four conrers on the image, usually 2 is the most, try it out!
take care and regards
Clouds over the Catalinas (72)