1 - I'd say it's worth looking into it if wildlife is a big interest of yours; I've sometimes found myself wanting more than my 70-200, but it's not essential. One issue IMO is that the 2.8 is so heavy, which could be a factor if you're going on long hikes, how many other lenses you're carrying, food, water, fitness level etc.
2 - Can't comment, I don't really have much interest in macro. I guess that depends on what you want to shoot
3 - Will have to let others comment on that.
I've been up to Sequoia/Kings & Zion, but not Death Valley. Personally I really enjoy Zion much more than Sequoia; it's much more spectacular and varied IMO. If you're interested in the big trees, Mariposa Grove in Yosemite has giant Sequoias. Depending on your time for Zion, I'd consider swinging by Bryce Canyon also.
I've used Yosemite Hikes
for a lot of great Yosemite hiking info.
Michael Frye also has a great photographers guide to Yosemite, both in book and app form.
Besides all the typical valley spots, I'd suggest going up to Glacier Point & Taft Point. I'd also go out along Tioga Road and see some spots from there. With 5 days, I'd also recommend following Tioga out the eastern exist and visiting Mono Lake, especially the south tufa area and the sand tufas at Navy Beach.