It's pretty obvious those are high contrast markers for cameras. Like crash tests, the footage of the J-15 landing is also going to be analyzed in detail and those markers highlight all the reference points.
The J-15 and J-16 both have retractable IFR probes but in J-10's case, I don't think its nose has sufficient unused internal space rfor mounting fully retractable probes.
Pretty accurate? You must be kidding me. Instead of four Type 054A (Jiangkai II) Frigates, there are currently twelve Type 054A (Jiangkai II) Frigates in service with five more under construction and three more on order (Source: en.wikipedia.org/wiki/Type_054A_frigate). For destroyers, there...