The plate is still masking the damage nicely

Would fixing it mean having to remove the interior?
And the 2nd problem....the cause.
Whats the most likely cause of the rusting as there is no point fixing it only to see it do it again. Could it be water getting in through the sliding window seal?
So 3 questions
How big a job is it?
Will the interior need removing to get at it better?
Likely cause?
Cheers