Ok, a lot going on here... so you seem to have partially resolved this Powerline thing by plugging the adapters directly into the wall as recommended. So, check on that one. That you get >100Mbps is a decent result, keep in mind the WAN port on Express is 100Mbps, not Gigabit. As others have stated, nothing beats direct Ethernet, but power line is a good alternative if it is impractical to pull cables through the walls. You might consider if there is a way to get both adapters on the same circuit, that tends to help and the adapters you have should get close to 1Gbps in an ideal world. One way is to run longer Ethernet from the router to Poweline Adapter 1 and plug it in to a circuit that is shared with the room the other adapter will be used in. But, again, if you are getting 100+ crossing circuits, good enough given the Express will never use more than 100Mbps over Powerline.
Back to how to connect the Express(es). Again, wireless connections between an Express and the rest of your network will work, but is not optimal. Try one Express connected to power line and see what that does.
As for Airport Utility, the article is a bit outdated, but still largely accurate. I have never had to reset my Airports to reconfigure them. When you have an Express connected via Ethernet (can do this directly connected to the router initially if it is easier), go to Wireless and set the Create a wireless network and provide identical credentials to the router WiFi, including the same WPA option. Next, go to Network and set it to Off (Bridge Mode). Then save and wait for the Express to restart and re-appear in the utility. At this point, you can unplug the Express and move it to any location you intend to test, and plug in the Powerline Ethernet, then the Express.
Another trick I use is to name the 2.4 and 5Ghz networks slightly different, NETWORK24 and NETWORK5G for instance. When ready to test speed, connect to the 5G network and run the speedtest. Then connect to the 24 and and test. Theoretically, you should see better speeds in 5G, but never more than the ISP speeds, and never more than 100Mbps if connected wirelessly to an Express (due to the WAN Ethernet limitation). To verify ISP speeds, run a test while connected direct to the router via Ethernet.
Unfortunately, current WiFi technology constrains you somewhat. Given 100Mbps Ethernet on the Express, and provided your ISP service is >100Mbps, any devices connected wireless or ethernet to the Express will max out at 100Mbps. But, when connected to the router's AC network, you would tend to see faster speeds. 100Mbps is adequate for most applications. 1080p streaming only uses 4-10Mbps at most. And for general internet surfing, 100Mbps is admirable speed. If you plan to download something large from the www, use a device connected to the router, and if wireless to the 5G band on the router.