I just spent about 45min creating a list of only four items with text and everything, then pressed save and the bloody page just showed the loading cursor. This was the first and last mix creation I'll attempt :P

Can you create a mix and edit it after the initial save, to add or remove items?
And if this is possible, where in my profile would I find it?

Yes, you can. That I don't know. Since GOG made certain changes to them (as far as I can tell they'd completely back out of the whole GOG mix thing if there weren't a few popular mixes that a lot of effort has gone into and a few users who still like making those) I have no idea where you even go to make a new one or where you can find your old ones.

Thanks, I just went into i.e. RPG Games list and on the right hand side are lots of featured mixes. Click the '+' Symbol and create your own. Maybe this feature is then already disabled and you can actual not save them?

You can't find it in the profile, you have to save a bookmark to the mix. There was a page listing all your mixes, but that was removed so long ago the story of it has almost become legend.