Rhino 8, 2025 (1 month away from 2026)
Still cant double click on a nested block…
I recommend that you install the BlockEditNew plugin via PackageManager 
It will add this feature (among many other):
This is a good plugin, does the job, but still not perfect. I still like the vanila way of double clicking a block to initate block edit mode.
All we need is just a double click inside a block edit to edit a nested block. Right now it is impossible
Yes, it would be great to have this in Rhino 9. It’s a small feature with a big impact when working with blocks. I’m still surprised this long-requested feature doesn’t exist yet.
I love this plug in, but unfortunately both it and VisualArq start to misbehave quite a bit when both are installed in the same machine…
Thus had to uninstall it. 
I had the same issue, so I removed it too.