I'm not entirely sure whether you asking about watching fireworks/PtN from along Main Street, in the Town Square Area, or up on the railroad station platform.
I doubt you'll find a spot up on the railroad station platform for the fireworks if you wait that long to get to DL. I think you'd need to get up to the platform hours in advance if you want a good spot. In terms of waiting until PtN is on the route to enter DL...the short answer is "yes", you can get into DL and probably find a spot on Main Street to watch the fireworks after the parade finishes. That being said, you probably won't find a prime spot because all the folks that are watching the parade will also be moving out ahead of you. In terms of after fireworks, if you head directly to the curb after the fireworks ends, you stand a good chance of getting a curb spot, but it all depends on whether someone gets there first or is watching fireworks from that spot and decides to stay put.