1. There is a standard damage system. Without body armor, you just die.
2. No. AI's do everything automatically. If the medic is in the same squad as the wounded, the distance irrelevant. If in different squad, then the distance is in the settings.
A Medic is any unit with a Med Kit. If the advanced mode is not enabled, then Med Kit and IFAK are no different, except that Med Kit is used indefinitely. If the advanced mode is enabled, then Med Kit, can restore health, and IFAK, no.
Based on this ... Almost any unit can help, but only a medik can restore health (unit with Med Kit).
The mechanism of work, the nearest unit is selected to provide assistance. If he is injured or dies, the next nearest unit is chosen. Etc.
If the unit is standing, has no bleeding, or does not have the means to stop the bleeding, but has low health, then the nearest medic is selected.