Great to see there’s interest in this! I’m not aware of much Python usage in mobile apps but yes that’d certainly be good to be more efficient.
This is based on the Sustainable Web Design model, which attempts to quantify server, network, and client device energy use for websites. Source data is my own crawl of the web + HTTP Archive dataset + CrUX report, I can share more numbers if it helps 
If there was no Django – I presume the majority of those sites would still exist, built with other tools. The emissions would be for those other tools to worry about (and yes could be lower or greater).
Re Rust, yes, it’s pretty clear switching to a more performant programming language for equal use leads to lower energy usage. Most CPUs consume less energy when idling than at 100% utilization (though not nothing), and if CPU usage is lower it also makes it possible for sites to provision a server with less beefy hardware.
However this is one of many factors, not a silver bullet.
In a web environment, maybe the need to run a database may take the lions share of resources anyway?
Yes that’s occasionally the case, depends lots on the architecture. The database often is “always on”, while web servers can be scaled on/off according to traffic.
for shipping speakers and audiences to conferences by aircraft there’s no carbon-free alternative
well, I imagine you’re aware of this already but online events and travel by train are clear alternatives