Multiple instances of lifelink on the same creature are redundant. Activating the last ability multiple times won't cause you to gain more life.

Activating the ability that turns the Keyrune into a creature while it's already a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.

Until the ability that turns the Keyrune into a creature resolves, the Keyrune is colorless.