1. It's the other way around, I want multiple objects as a mask (so without having to expand it, that's why I mentioned non-destructive) and I'd like the area of the strokes to count in the mask.

If you choose a group it just doesn't work. If i make a compound path than it's destructive.
And the second point about the stroke is the request. I know it doesn't work right now, maybe it's a pixel based effect and that's the restriction, but maybe the solution would be to have a pixelbased mask option thing.

2. Opacity masks solves the group masking and strokes, but if I'd want to use the same layer to blend, I need a duplicate in only white. And illustrator doesn't have instances.
So I need to do the same edit in both copies of the shape or group, plus accessing the mask through the transparency menu (which is already a bit of a confusing UI)

It's just so much simpler to think of this in a Photoshop way, or even better After Effects.
You choose a group or layer and say that it should be masking another group or layer, still having the option to keep it visible or not.

At the moment Ai allow several methods to achieve something like this.

1. Select an object, highlight a layer you want to clip (but only one), and from the Layers panel’s menu choose 'Make Clipping Mask' command (you would also have to have the layer highlighted, otherwise Ai won’t enable the command). This will clip the whole layer.

Basically it’s the same thing as clipping a selection, but it just allows to add more art into the layer without isolating a clip group first. The order of the clipping object in Layers is not important.

If you choose a group, then Ai will intersect all the paths in it to calculate the final clipping path.
If you pick a compound path to clip, it would behave like a filled compound path, according to options chosen in Attributes panel (paths’ directions and fill rules used).
With this method the appearance of the path means nothing, so the stroke won’t contribute. Ai uses geometry only to clip, not the rendered appearance (this includes strokes and effects), and it won’t change ever, I believe.

2. Use opacity mask.
Target the layer itself by clicking the circle marker next to its name in Layers, and click Make Mask button in Transparency panel.
Enter the mask by clicking the thumbnail on the right, Now the art you draw will mask the whole layer, same as with the clipping mask in the method above — but this time the stroke and the effects matters. The image would have to stay black on white though (or vise verse, with Invert Mask option checked.

In both cases you can use live text.

Are you familiar with these? If yes — why not use them and what would you like to change and why?
If no — well, same questions (after you try them).