Thanks for filing the issue!

@Reporter: As we are not very clear about the statement "But alas, disabling it on B also disables it on A, C, D, E." Could you please elaborate on this, Does A, B, C... here mean different machines where single account is logged in?. Any further inputs from your end may help may us in triaging the issue in a better way.

Yes, different machines where single account is logged in.

Also in such cases the user has no way to just try an extension on his one machine, without immediately deploying it on all his other machines, or having to
sacrifice synchronization of all his other extensions by turning off synchronization.

Thus maybe on the chrome : extensions page you need little "synchronization" icons next to each extension, their default state being "yes, keep me synchronized", but can also choose "no, don't synchronize me with my other machines", and even "synchronize now".

Also note that case where the user just wants to try an extension before committing to also install it on his other machines.

Here there should not be a race condition where the user needs to install it first, and then turn off synchronization. There should be some way to avoid that one minute of worry.

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Thanks for the feature request! 

Redirecting the request to the Extensions team.

As per comment #6, this is a Feature request.
Hence marking this as Untriaged for further updates from Dev.

Thanks..