1°) Probably a mockup, I'm agree. If it was a real one, PLAN would probably use a "yellow one" : I mean with yellow paint and test marks on the cabin (as the J-15 n°555), which is currently use for take off and landing tests from a land base. For the first real take-off & landing tests from the Varyag, Ingeneers must verify if the structure of the J-15 resists to shocks with fly deck. To summarize : this J-15 is not prepare for tests.
2°) Agree also : it's not the first time that you have a J-15 mockup on the fly deck. A J-15 mockup have already been observed severals weeks before (with a Z-8 mockup moreover). I don't remember exactly, but I think in march or in april this year (during spring 2012, sure).
3°) Presence of J-15 mockup is not real surprising. For example, during the modernization of INS Vikramaditya in Russia (ex-Gorshkov, aircraft carrier bought by Indian Navy, currently in sea trials), a Mig-29K mockup have been installed on the fly deck. However, it's true that this mockup have been removed for the sea trials...
4°) Difficult to say if the PLAN will take "the risk" to make a landing and a take-off in the same time. If think that they would probably install a J-15 on the fly deck in Dalian (with a crane), take the sea and make a take-off. Landings needs more time, training and confidence (in the plane & the pilot!).