Well, I have a multipul installs of dotnet from 3.5 to 4.6 here on 7 win10 has 5 and does not run other versions.
v3.5 works on v2, and v4 on v3 the latest dotnets should work as far back as 2.
As for visual c, they shouldn't conflict as such because certain programs need certain libraries.
Now, my only wish would be that windows does what linux does.
The libs don't take up that much space but even so.
I'd prefur say you install something or try to do something.\
Say I want to run x program but its missing y z dll.
Windows is unable to start this program because *.dll is not loaded please reinstall.
To avoid this I load just about every library I can to stop that error.
It happens with codecs.
If I am using linux, I type in something to install, it says this needs such and such component, do you want to install it or it tells you what to do.
I have often installed programs for ssh and a few things on linux, and got messages like this program is not installed, or this program is part of another install for servers, or in deed, unable to start because this thing is missing.
However in linux you can then install it because it basically tells you what to install.
If ms would do the same at least with its libs then we wouldn't have this issue better still with linux it tells you what is needed, what could be nice, what is not but if you need it it would be nice or simply whatever.
In windows, I have slim dx, java latest, klite codec, visual c 2005sp1 2008sp1 2010 2012 2013 2015 2016 and their 64 bit equivilants.
On the subject of python, both 3x and 2x are still supported 2.7x is the version a lot of stuff uses however its a good point a lot of stuff has moved on from the older releases.