A few notes:
1) 10 acres would normally require a 24" minimum culvert for a 25 year rational analysis in central Florida. Unless there was percolation, a very long time of concentration or significant storage on the order of 0.5 acre foot, more or less.
2) If you do not decrease the storage capacity of the ditch or increase the flow to the ditch or decrease the flow time of the ditch, your culvert/driveway will no create any flooding to your downstream neighbor, even if it overtops. However, if your downsteam neighbors' culvert is too small, he may flood you, the upstearm neighbor, by creating a tailwater condition for you. Therefore by undersizing your culvert, your impact your UPSTREAM neighbor, not your downstream one. (Rule of thumb - increasing your peak runoff may flood downstream, creating a flow restriction may flood upstream)
3) The DOT has soverign immunity for any errors of judgement, you and your firm do not. Just because they approve a deficient design, does not make it OK, nor does that protect you from liability due to negligence.
4) I did not mean to imply that you would "cheat", however, being human and probably much older, I have had the temptation on occasion. It is not cheating to interpret data or estimate parameters favorably, as along as the interpretation is reasonable. For example, if you can use a C factor of 0.1-0.2 for undeveloped areas, use 0.1.(If your do not need to increase the undeveloped areas C for the recurrance interval, do not.) Do the same for your ditch manning factor, use a high retardance. For your culvert, use a low entrance coefficient due to the proposed beveled edge, etc... Did you consider percolation in your ditches?
5) It is good to talk to the regulators for their input. Also it is good to work on a problem until you are comfortable with the solution. You must be 100% comfortable with all your solutions.
6) Sorry that my advise may seem contradictory, but additional information may change my perspective.
best of luck.....
Clifford H Laubstein
FL PE 58662