RE: Weeds & plant development?
Can you find a varity with a 100 or 90 day maturity date?
I am not an agronimist, but the maturity date and shattering date you described are probablly two different dates. Controlling weeds will help the plant's growth and production, but my guess is that it will do little for maturity. Sunflowers are a warm season type of plant, so planting too early (and I have no clue what too early is) may not help and even hurt the stand. I don't think sunflowers are very frost or cool weather tolerant.
Therefore, if you can't plant them earlier, then try to get an earlier maturing variety if possible.