Hi, I am the guy who wrote the previous post you refer to. I have been doing further checking on your questions. Let me start by saying if before you had your Echo your data usage was fine (even if you did occasional streaming of media from TuneIn, Netflix, etc.) you will be fine with the Echo.
When dormant, I cannot detect any significant usage according to my router's traffic monitor. There is a tiny baseline constant flow of less than 1 KB/s as all the iPhones, tablets and other devices check in with their Mother ships. In fact, the iPhone is a big data waster and elsewhere on this site there is a thread about that. A big data hog is all the app notifications going on. Turn off those things that are not essential. Google internet to find articles about conserving bandwidth of an iPhone for all the tips.
As for weather requests, flash briefings and other Alexa requests -- not a problem, since they use little bandwidth and are very brief. Don't be concerned about them.
The biggest data users are streaming media such as TuneIn, Pandora, etc. But, as I said above, if you have been streaming media prior to Echo, using the Echo for these tasks now would not change things much. since 20GB plan seems fine for you up to this point.
An update on my TuneIn streaming. I checked different stations today. All were different results since TuneIn stations all have different data stream rates and the faster streams have the best audio (especially when heard via the stereo). The highest rate I saw was 40KB/s; another was around 12-15KB/s. Both stations we PBS music stations. Anyway, leave you Echo plugged in and use your Echo, and compare your data usage from the past month to the upcoming month. You may not see a significant difference.
I have my Echo plugged into a wall on/off switch for convenience, you might want to do same.