Linux: Dynamic switching between Nvidia and Intel GPU in a Laptop

How well does this work at the moment? (with the proprietary Nvidia Driver)?
Can I “activate” the Nvidia GPU only for some application without having to reboot every time? (no, I don’t mean Bumblebee)

Nope. Work is being done on this, proposals for X server extensions are in the RFC phase but no other Xorg developer has shown interest in this so far. Don’t hold your breath.
https://www.phoronix.com/scan.php?page=news_item&px=NVIDIA-EXT-Vendor-Select