Well there are two points here. First F35 vs F18
My hunch is that if the Chinese put their Fifth generation fighter on their next AC (which I feel is very likely) IN won't go for F18. The whole strategy of IN is centered around the concept of having qualitative edge over PLAN. Since we can't match quantitatively. Then the second point kicks in which is will US ever give us CATOBAR/EMALS without their fighter. Again my hunch is a big NO. So unless we are going for another STOBAR carrier in IAC 2, F-35 has a good chance. But again this is a hunch.
The relations you make here doesn't fit, because if IN is afraid of a Chinese carrier with a naval fighter soon, their first option is not F35C, but N-FGFA! They know FGFA is more capable, they already know how to develop STOBAR carriers and with the experience of IAC 1, it's much simpler and faster to go for a bigger version of it. F35C on the other hand will be not available for IN anytime soon, since it's already delayed and most partner countries and navies needs to be supplied first. Not to mention that we need catapults to operate them, which then again means, developing a new carrier...
I am totally with you that catapults will most likely come in combination with a fighter, but then IN has the choice to take it or not, since they have to evaluate if the the operational advantages are worth the restrictons of weapons, techs, or performance by US laws and that for both fighters! The restrictions for F35C are logically higher than for F18SH, which means catapults and SHs might be a compromise that IN makes, while developing a naval AMCA for catapult operations in the long run. F35C and catapults on the otherside means high restrictions and costs, which then might not make catapults worth it anymore, so it's not that simple and several different things has to be considered.
Imo, the F35C has very low chances, because IN can't afford to be so operationally limited by a foreign country, especially when the aim is projecting power! So my prediction is either F18SH as a compromise and in combination with AMCA, or FGFA and N-LCA on a STOBAR carrier.
And when have the Chinese shared their concrete plans. Every thing that comes from their end is a surprise - DF 21, J20, J15...i can put down a long list.
Because people still tend to underestimate the Chinese production capabilities! I also think that J15 is basically only a training aircraft for them, to gain experience in navalising and carrier operations, while the long term will be naval J31s, which might be even developed by selling them to export countries and make them cheaper againt.
China is very simple and pragmatic in such things, unlike us, that's why they can pull out such developments much faster than we can, since no pride reasons of the manufacturer, or the force will pose a problem. But that's exactly why I can't understand IN or many Indians putting their hope on N-LCA, rather then on developing a naval NG fighter as soon as possible?