Well thank you again for that tutorial! There's a lot that I wrote as I was writing as I went. However, there's a summary at the very bottom if you don't feel like reading through all of this haha.
Small bit of back story before I mention the results - I can have a successfull connection with the modem using a PPPoE connection and the router in bridge mode. Of course, my intended goal is to have that flip floped. So when I started your tutorial, I actually had the modem/router settings set up in the connected-but-unwanted way (modem using PPPoE/router in bridge). I took everything step by step and transitioned step by step so as to be able to provide you with all the details, so sorry in advance about the length of this post.
Now as for the results - I followed your tutorial in the order that you had mentioned it. When I did it that way (with the modem being directly connected to the Mac) the PPPoE connection that was setup in the Network settings worked. I then put the modem into bridge mode. After I did that, it still worked.
So then I decided to add the router into the mix. That's when things went off the cliff. When I plugged the router in, a pop up said, "You were disconnected because the PPP server is not responding. Try reconnecting." I clicked OK then continued into Airport Utility to change the router to "NAT and DHCP mode." After I did that, I applied it, restarted the router and then got the "Double NAT" error. Despite the error, and just to cover all my tracks, I still tried connecting to the PPPoE connection that was made through your tutorial and it failed (not to my surprise).
I then went back into the router settings and changed the "Connect using" from DHCP to PPPoE. After this, the router had no more errors and gave me the green light. However, the same problem was still there where despite this, I still couldn't connect to the internet.
Finally, I went back into the Network settings and tried reconnecting to the PPPoE network that I created through your tutorial. This resulted in it giving me the message "Could not find the PPPoE server."
Something did perplex me, though, and it lead to me solving this problem (I think!). If I had the modem was in bridge mode, but the router connected using DHCP and in bridge mode, I could connect to the internet this way if and only if I used the PPPoE connection made by your tutorial. So then I was thinking - if I put the router as connected using PPPoE, it will automatically put it in DHCP and NAT mode. However, what if I kept it connecting as DHCP but had it in DHCP and NAT mode? So I tried this and the PPPoE connection made through your tutorial went down, I had no internet and got the "Double NAT" error. Then I just dialed it back to just DHCP only. I clicked update but it gave me "invalid value - Sharing a range of IP addresses using DHCP only requires manually configuring your WAN IP address." So I raised the starting DHCP range from 192.168.1.2 to 192.168.1.100. This let me successfully updated and it connected! I am just not certain what I should do for the DHCP reservations as I do need to open up a few ports....
Summary: Here's how the end result looked - The router was configured to connect using PPPoE, was in DHCP and NAT mode, and the device itself had the green light. The modem was configured in bridge mode and on the device itself, the power, ethernet and dsl lights were all green. In the network settings, the ethernet was stated as connected, the PPPoE status was statd as not connected and I have no internet.
However, I CAN connect to the internet using the PPPoE connection I made through your tutorial if I can have modem in bridge mode and the router in DHCP only.