Sorry for the delay. Thank you for the project file.
2) We've improved deleting keys, which is now very fast in 3.7.19-beta (though not yet available). Selecting keys is also much improved, however selecting many thousands of keys can still be slow. Your animation has 31,000 keys, which is quite a lot of data for Spine to deal with. It should not be an issue most of the time, but is likely to be sluggish for some actions, such as box selecting all keys. It should never take minutes though, so if you run into that again please let us know and likely we can improve the performance of the actions you are taking.
5) You are trying to export a video that is 13120x18442. It would take over 950MB of GPU memory to render a frame at that size. The error message is shown because your GPU fails to allocate such a large buffer. I'm not sure any GPU will handle such a large buffer, as they have both max texture dimensions (in pixels) and a max texture buffer size (in megabytes). If you had a GPU which could export such a large video, you may still have problems playing it.
You could scale down your root bone before exporting to reduce the video resolution. In the future we plan to allow specifying a "camera" frame which will crop exports. Until then unfortunately it may be difficult to export a large cutscene to video. You may be able to use a clipping attachment to crop your cutscene, similar to a camera.
