We fly frequently with our kiddos - 20 mos and 4.5 yrs and never preboard. In fact we are almost always the last to board so that the kids are cooped just that litttle bit less.
Installing the car seat is pretty easy. I take the kids and gear, husband takes the seat and walks on before me. I fold up the stroller and wait on the gang way for a little bit and then wait towards the front of the plane while he puts it in the seat. Takes no more than 5 minutes.
As for the first part of your question - Delta no longer offers pre-boarding, but if you asked the gate attendant they might allow it. As they say, nothing ventured, nothing gained.
Happy flying!