About leaders dying: There's just going to be a ton of field promotions. As there would in the real world. This should also make the leaders worth protecting and maybe discourages them from doing anything reckless. This could also mean that our battleplans could change drastically in the middle of an op if platoon lead decides to take a bullet.
People shouldn't be tied to their earlier roles. That would only cause problems. In my opinion it would be best if either HQ or Platoon gave an order about which element they want as reinforcements next and then people will either start filling that out as they respawn or slot up in the manner Jash described. I myself don't see a reason why the rank of your previous dead character should matter in the selection order, but would rather keep it based on who was first (they had to wait the longest anyway). This way we could get more variation in our roles and maybe even test roles we wouldn't otherwise.
Also about your dagger driver example: There shouldn't be any problem of you choosing dagger driver if Platoon wants to get another dagger to reinforce while the other one is still on the field.