Why doesn't Google+ auto upload all my pictures consistently and in the right sequence? Google+ doesn't upload my photos consistently, and sometimes it uploads them out of order.

Why would the app not do its job consistently? There are missing pictures, and then a week later they randomly show up (sometimes, sometimes not), and sometimes out of sequence, which is very frustrating. 

If I uninstall and reinstall Google+, will it have to reload the last 4 years of pictures? Or worse, lose my pictures from my old droid that died? I have plenty of storage space left (several GB). 

It is a new Samsung S5 phone, but I have the same problem on the old Droid Razor. Auto upload is enabled, and if I try the "upload now" option, it tells me everything is already uploaded, when I can clearly see I am missing pictures.
Maybe because you choosed "Only WiFi" when the app asked you if you want to upload photos via wifi, mobile or both... 
Try clearing G+'s data and restarting the settings step from zero...
I had a similar problem with a S4 and this was the problem/solution ;)