Is GWX technically malware? [closed]

I would call it a rather irritating annoyance, but not malware - neither technically nor substantially.

installs itself without the user's consent as an "important" update

Only if you have "important" updates set to automatically install. Otherwise, you have the chance to review the list of pending updates, and uncheck (or hide for good) KB 3035583 which installs the "Get Windows 10" applet.

Needless to say, once you did enable auto-install of "important" updates, you waived your option to choose, and left it to Microsoft to decide what's "important".

actively prevents the user from disabling it permanently

There are several well documented ways to get rid of KB 3035583 if you installed it by accident, see for example How to disable the “Get Windows 10” icon shown in the notification area (tray)? or How to remove Windows 10 upgrade updates in Windows 7 and 8.

there is no objective definition of malware, so as such you are free to consider it what you want. The best definition I've found is here at the Oxford English Dictionary, but it includes subjective elements.

The code security industry has to make decisions on what is and what isn't malware by their assessment, as do legislatures around the world, and while those definitions may contain terms that describe GWX, the vast majority of users do not believe it to in fact be malicious, so no major entities are likely to determine it to be such.