Ronan is not immortal. As seen on this Wikipedia page, his powers are listed as:
Expert in military tactics
Skilled hand-to-hand combatant
Superhuman strength, speed, endurance, and reflexes
Utilizes Kree exoskeleton armor
Wields "Universal Weapon"
It mentions nothing about immortality, nor does the page for the Kree. ...
As a Kree, he is well above average strength. As an Accuser, he wears Accuser armor, which is meant to enhance the wearer's strength and stamina. At the beginning of The Guardians of the Galaxy, Ronan executes a member of the Nova Corps, and the victim's blood mixes with what is likely Odium. As seen in Agents of SHIELD, Kree warriors will inhale/ingest the ...
There is no set "leader"
Peter Quill is the nominal "Captain" (and thus "in charge" in his mind) because it's his ship they are riding in but these people are together by choice and decisions, at least from what we have seen, are made as a group.
They all have their input and if one of the group disagrees they are free not to take part.