then you can stay until March 15th. If you have an activity on March 16th, you could apply for an extension of stay without the required documentation. They'll deny the extension, but you'll get 7 days of leniency.
Do it on the last possible day, which in your case is March 14th. You'll get 7 days starting that day, so then March 20th will be your last. So it takes you only 5 days over the 15th, but if you're helped out with exactly that, just go for it.
then I think just making something up is the way to go. You can't get refused for not sticking with a flight plan for a visa that doesn't require a booking.
You don't need to leave within 3 months, the visa is aimed at staying longer. Is the return flight a required field to fill? If not just leave it blank.
to say you don't need one, you should maybe not have presumed that.
If you stay for a year or more you do need a visa. All in all you give very little info though. Like: what purpose are you staying for? Obviously if in November you came without a visa and now you want to stay for a year something relevant has changed, and it is a little strange you ask Brandon what that is. He didn't know, but you did.