excellent work Seymore!
the lagoon was my first ever deep space pic also
and you have done quite well here.
i wish i got onto flats earlier on, they just make such a difference to the end result and make processing easier. have a look at this article
http://www.iceinspace.com.au/63-211-0-0-1-0.html in particular the flats section. i use twilight flats, with a canon they are pretty easy, you just need to make sure you get it at dusk or dawn, set the camera to "Av" and aim at an even patch of sky. try and get as many as possible.
some people might not agree but i will say yes you can capture flats now and add (subtract) to this image. the camera alignment needs to be exactly the same to when you took them - if more dust has been introduced since you took the pic it could affect the image somewhat, but worth giving it a go.
Russ