Here is what I did. Not sure if it was the two step process or what. I did a DNN 4.9.4 to DNN 5.0.0 upgrade. When I did this, I copied in the new files and browsed the site. It just kept processing the page so I let it run for about 15 minutes and then the page stopped and nothing was there.
Then I tried to browse the /install/install.aspx?mode=upgrade and let that run until the page actually came back with the DNN upgrade results. They were all a Success. Then I had a DNN 5.0.0 site that was functioning fine.
So I copied in the DNN 5.1.2 files in and browsed the site. I let it run about the same length of time and it finally came back with a blank page. I looked in the db and saw the version was 5.1.2. I got off the server and browsed the site on my workstation and it came up. I logged in as host and did not have any errors. I noticed that I only had the single Google Analytics module on the site, whereas when I did the single upgrade all the way to 5.1.2 I got the errors on every page and had two version of Google Analytics on the site.
I cannot say exactly what the best direction should be. But I know that I just waited until the page stopped processing totally before trying anything else. And like I said, it took around 15 minutes. On the other upgrades to DNN 5.1.2 I did them in one step and they worked fine. So I would say to be persistent and always have good backups.
Cliff