1. I don't know if I am right or wrong in this. But imo Tejas Mk.2 will feature 110 kN class engine and that engine is to be built by joint effort of India and France. AND for AMCA, it will be 125 kN class advanced engine which is to be built by joint effort of India and UK. It just speculation, don't take it seriously. AMCA needs more kN as DEW in Mk.2 will require extra power.
2. How am I more confident? My gut...it is my gut.
See, it is not very hard to analyse F-35 is bulkier than AMCA. AMCA is more slick in aerodynamics. Because it will be built almost last among all stealth fighters, its designers will be fortunate to study different designs beforehand and to finalise the best shape for our IAF requirement. I am confident about RCS material. Although it is universally acclaimed that F-35's situational awareness is unmatched for now. Its computing capability, DAS and information sharing capability across all military platforms (Air force, Military and Navy) is noteworthy. If we can even attain 75% of those capability, AMCA may become like a medium class F-22 with a technology of F-35 which will be scary.