lol! Take friendly matches serious at your own risk.
Most key players don't take friendly matches seriously and some of them won't even come in the first place and you want to try your luck with a second squad of a term playing with another's first square? If you've watched most international friendless you will observe the casual play from most teams as they mostly use friendlies to check out their players to see if some are good enough for the main tournament.
Friendly games is better staked on goals and not direct win. You have stated correctly that key players don't play most times and when they do they are careful and the coach substitute them early in the game to avoid being injured because they could be target and subject to injury.
The question is why play a key player when nothing is at stake only fitness and to taste your strategy of which the key player can fit into the real game since he is already fit or he can do the fitness exercise somehow not necessarily on the friendly game. So the best betting option that I have successfully tried is on goals because defenders too are not keen on defense, nobody wants injury on friendly game.