If I had to skip one it would be Canyonlands. I wouldn't skip Zion. Topography is so very different there. For you are down looking up the whole time. Many easy/moderate hikes there. We just went in October. Such a memorable trip. We hiked Narrows at Zion. But even a hike down the mile trail to where the water starts is worth it! My favorite parks were by far Arches and Bryce. You will love it!

No, you don't need a guide for these hikes. (I don't know about The subway). For Narrows you might need some equipment that you can rent near the entrance to the Park. Emerald Pools is an easy hike if you start from the Lodge and finish in Grotto.

I think it will depend on your ability and determination. I am 61 and have a bad knee but I am in good shape otherwise. I took lots of breaks and my daughter helped me down the steeper parts of the upper pool trail (going down is harder on my knee). The lower pool is relatively easy. The upper pool is considered moderate and you will have to climb over rocks. It was doable and well worth it -- the upper pool is breathtaking. We went from the Lodge to the lower pool then through the middle pool to the upper level. Coming back we went to the right to Kayenta Trail (very easy and view of the valley is unbelievable). We made a mistake and took the first right (marked to the Grotto); that trail was a bit steep. I think if we went lower and took the second right (to Grotto) the trail would have been easier. But ask a ranger at the Visitor's Center; they are very helpful. This hike was one of our highlight so I would say try it and you could always turn back. Good luck!

Lots of shade to the lower pool, more exposed on trail to upper pool until you get there. Connecting trail to Grotto from Emerald Pools is exposed to both sun and steep drop-offs. Bring water with you and wear a hat/sunscreen. :)