As of F-18 failing Leh trials....thing is, pretty much all foreign products fail those trials. Their engines are not meant to operate at those altitudes. It's not about the cold, its about the altitude - the thinness of air.
F-16 failed the same trials. Even Su-30s and MiG-29s, back when we first bought them, had problems operating from Leh. Only modifications to the engines and change in the composition of fuel used could fix the problems.
Even LCA suffered in Leh when the GE engines shut off. It was modified later ofcourse.
It's a perennial issue because of the environment, nothing wrong with the aircraft per se.
Coming to the choice of plane, it literally makes ZERO sense to buy any other plane except additional Rafales. If we don't have money, then buy fewer. Buy more later when funds are secured. Compromising on quality or capacity due to short-term monetary problems will prove to be a grave error.
There isn't any outright merit-based reason why either of the two US fighters need be selected. If we want the best capability-wise, Rafale or Typhoon should be your pick. If you want something that's cheap to buy, MiG-35 is your bird. If you want something that's cheap in maintain in long run, Gripen comes out on top.
Only reason to buy F-16/F-18 is if we want access to F-35 in future. The US knows this very well too. That's why all the constant "India's Road to F-35" marketing campaign from the likes of Lockheed.