According to my information:
- If your application is for WP7, it will certainly be available for both WP8 and WP8.1 until you provide a separate package for later ones. so your first question is yes
- , 8.1 sdk WP8.
:
WP8.1 WP8,
.
, WP8.1 . , . - WP8.1 . , WP8.1.
.
WP8.1 Windows 8.1, Universal Apps Difference WP8.1 Win-rt Apis WP8.1 Silverlight, .