I think you might have uncovered a bug here. This does indeed work, but in some cases, the index is recreated but never reloaded - it ends up using the old index, which is obviously a bad thing.
Could you try this please - in tk2dEditorUtility.CreateIndex(), add the following lines at the very end of the function
index = newIndex;
tk2dSpriteGuiUtility.ResetCache();
Once you import the new sprite collection package, invoke "2D Toolkit > Rebuild Index". When you create a new sprite, you should see the sprite collection you imported in there.
What worked for me -
1. In my original project, I simply select the _data project and export as a unitypackage
2. In my target project (blank project with only tk2d in there), I import the package
3. Run 2D Toolkit > Rebuild Index
4. Create sprite in viewport, and the sprite collection is available. The sprite preview is pink.
This worked in both 1.7 + patch2 and 1.75 beta 3. 1.75 beta 3 has a bit more code to handle a few more cases, for instance when the objects do not exist in the project, but only at runtime. This shouldn't affect you, though.
Let me know how it goes for you. In any case I'll be adding this fix to the codebase.
p.s. Even if the sprite collection data object is deleted after this, you shouldn't get an error in the view