Welcome to Spine!
1) The runtimes version must match the data version:
2) Yes, run the old version, import the data, save a project file, open the project with a newer version, and export it. You can write a script to do that if you have many data files.
3) I don't understand. To load image in Spine, they need to be individual images. At runtime they are usually packed into a texture atlas. If you have only a texture atlas, you can unpack it.
4) I don't understand. The version number on the runtimes license is separate from the runtimes version. If you want to use 2.1.27 data then you must use 2.1.27 runtimes. That version is ancient and I doubt there is a Spine web player for it.
3.8.99 is also very old. There is no reason to keep using 3.8, I highly suggest using the latest, currently 4.1. The Spine web player is newer, has many fixes and improvements, and will work better.