Your brother needs to really consider whether doing another degree will lead to additional benefit. He won’t qualify for PGWP again so how will attempting to study again get him closer to PR which is assume the goal? He is probably better off returning home to work on increasing his CRS score...
Example of a recent judicial review where parents H&C was denied. Filing for H&C for parent(s) can come with significant longterm consequences if denied. Please think very hard before going down this route...
At the Visa officer’s discretion whether to overlook the 3 weeks. These days tough to get away with shortfalls but nobody can guarantee what will happen.
Would consider withdrawing the application and applying again. Drastic but seems the easiest option if there are no answers. The situation in Iran and risk of not returning home will always be an issue but has gotten worse.
Your boyfriend should show he can fund his own visit and has strong ties to return to his home country like longterm job, property, etc. Previous international travel history, especially to certain countries, also helps.