Can I change my FastPass+ selections on day of my arrival?

Yes, you can change your FP+ on My Disney Experience at any time until the end of the window. So, if you have a FP for Dinosaur from 9am-10am you can change it at 9:59am without a problem (as long as you don't use it).

Once you use your first 3 FastPasses you can select whatever is available in My Disney Experience at any time. If Dinosaur and Primeval Whirl are available in the afternoon then yes, you can grab those one at a time. After your first 3 you are only allowed one at a time so you select one (a 4th) then, when you use that one you can select another (a 5th), and so on.