1) No, there isn't. It could be a nice contribution. 😃
2) You can do that, no problem. Well, the only problem is defining the attachment SRT which is relative to the bone. You can create a Skin and RegionAttachments programmatically. See the spine-c API, especially where SkeletonJson sets the attachment SRT, and also AtlasAttachmentLoader which initially creates RegionAttachments.
3) What doesn't work about it? You can type a relative path, which is relative to the project file directory. Try holding shift while you click to open the file dialog. This will try to use an alternate file dialog.
4) You can't yet. This will be added soon:
https://trello.com/card/playback-option ... c0059d9/17