According to my knowledge:
- If your app is targeted to WP7 then it will be surely available for
Both the WP8 and WP8.1 until and unless you have not provided the
separate package for the later ones. so your first question's ans is
Yes
- You said thinking about targeting the 8.1 sdk for WP8 But before
making this point explore that - Is it really worth or fruitful to do that
because what is New to WP8.1 will not be available for WP8 so
what's pushing you for doing it.
As you already mentioned that WP8.1 devices are very few. I think this should not be the point of making decision. What your decision should depend on is - Are you going to use new features of WP8.1 in your app or not. If you are doing then you should also target WP8.1.
Because in the end every single user should have better experience of your App.
If you are trying to use common code for the WP8.1 and Windows 8.1 then you can also check for Universal Apps and Difference between WP8.1 Apps using Win-rt Apis and WP8.1 Silverlight apps because their are significant differences between the later ones.