I don't think that completely correct!
Both Su-30SM & J-XX apprently use high percentage of carbon composites. They are not drastically redesigned. Strengthening the airframe to carry a Cruise-missile should need more changes, & we did that (before that i assumed that we aren't allowed to do modifications).
Su30 was built to be a heavy fighter. So carrying heavy missile/bomb was inherent to its ability. It was already meant to carry Klub class missile which weighs 1.5-2 tons. So, modifications to carry Brahmos was less significant than what you claim.
not necessarily. the fly by wire system variables can be altered to provide different continuous correction input to keep the aircraft flying within the stability parameters even after altering the airframe materials.
FBW can be tweaked only to some extent. It is not that any plane can be made to fly by FBW. FBW only optimisies the aerodynamic forces but the design and weight balance is also crucial.
It is true that planes are made to fly with weight variation when some pylons are emptied while others are carrying payloads and situation like wings getting hit by enemy fire. But the optimal aerodynamics of the plane will be hit badly. The plane will be able to fly but not do maneuvering like it was in its its right form.
So, replacing some parts with composites is difficult. India did get Canards fitted on Su30 and still the plane functioned without having to be redesigned from scratch but even that required good amount of testing and calibration before it was fit to fly.
Also, in case of composites, thinga like internal LRU, fuel tank etc will also have to be modified. This is unlike addition of canards which was an additional control surface. Arbitrarily adding composites to easy areas will serve no purpose. But changing internal structural aspects can be really time consuming. It is not worth it.