I'm not talking about universal apps. If developers mostly ignore the platform don't expect many Universal apps any time soon.
What apps will run in these devices? The merged OS is mostly based on WP, so WP apps should run, but probably they will add support for Store apps because WP apps aren't optimized for tablets. Although, Store Apps won't be at home in WP.
read my comment again
they will not give the exact same app
you cannot expect the version of an app to run without changes on the other, the universal app thing is an advantage for devs because they wont have to maintain two separate apps,just one app with two UIs,one for pc/tablet ,one for phone
you havent understood the advantage i guess,anyways its more of an advantage for devs,as a consumer you should just know that it means faster updates and releases
ill give you an example to put your doubts at ease
think of an ipad and an iphone ,now imagine say the FB app on both,same code but different UI
even the .ipa is the same usually
so one xap for both,different UI when on installed