Why are the options greyed out before I can download the game? Let the user choose the version, plain and simple.

Tried to download the game slightly prior to the official release, realized it was a 4 months old version, cancelled the download.
When the release was available, hit the download button but in the list the option was stuck on the aforementioned 4 months old version... Downloaded the whole 30 gigs only to see the update to the 1.0 version in the downloads section... Have to download another 30 now.... Fabulous...

For the same reason you can't choose to only install certain DLC: nobody implemented that feature. You can only downgrade and uninstall DLC after the installation is complete.

With the DLCs things are a bit different though, aren't they? 99% of customers want the dlcs they have to be installed. Whereas in my case I simply got the old version when the newer one had already been out and available, but I had no option to change it somehow. It's quite frustrating.

I believe the reason is
a) I've started and cancelled downloading the older version
b) auto-updates are disabled in my galaxy settings

But how can I warn others or is this a feature potentially wanted by many? Alongside the DLC problem/limitation you've mentioned?

It can also be argued most users want the latest version installed, so there's no point in prompting for which version to install. In your case, it was a brand new release and the latest version wasn't set as the default to install. Maybe that version was still marked as a beta for a while.

In the cogwheel menu in the top left, you have the Send Feedback option if there's anything you'd like to suggest to the developers.