Just to make some things clear:
This Jetstar cadetship does not provide the command hours required to obtain and full ATPL.
Until a junior F/O or cadet achieves the hours to hold a full ATPL they cannot receive a promotion to the position of First Officer ($37,000 base pay difference) let alone hold a command.
Jetstar is under no obligation to provide the command time to a junior F/O, nor can it be logged without the approval of the company for a training captain to supervise it.
Because there is a $37,000 pay rise involved there will be a disincentive for Jetstar to provide command time required for promotion until the supply of experienced candidates dries up.
Seniority is only a major consideration in regards to promotion to either F/O or command within Jetstar.
If you still wish to pursue this opportunity then good luck to you and I hope it goes well for you.