moses34

Whether or not you get a visa depends on two things. The first is where your case number is in the "queue" (random). The second is whether you personally qualify for a visa (non random). So you can't just apply some arbitrary percentage to the possible outcome. If your number is not too high and you meet all the requirements your chance is pretty much 100%.

There is no deadline. As long as they've not run out of visas and next September (2015) has not come (it might be August or July, but anyway you're alright).
Not sure about the birth date, I'd call them and ask to be honest.

Your English is fine, explain it how you have explained it to us.
Do not send in your DS260 later than four months before the end of the fiscal year ie no later than end-May 2015. Although there is technically no expiry date if they have not processed and scheduled you before they have allocated all visa numbers you may not get an interview.