Really should not guarantee things that you did not test yourself. The heal and shield work from % of health not amount, go play at Kakashi clone with Kabuto passive, it becomes obvious in one minute.

When everyone is full hp, it's similar to how passives/auto work, you can manipulate it by order ninjas are dragged on team.

From what i notice it prioritizes people who are first in buff order, also summons are counted faster then any ninjas in buff order, i have been able to manipulate it pretty easily with those 2 rules.