For the lowpoly it depends on how close you can get to the object. Normally these holes and the small gap won´t be part of the lowpoly geometry. A normalmap can be used for those details.
You have very high density, is that your lowpoly? or do you use turbosmooth? I keep my meshes as lowpoly as possible when doing highpoly so its easier and faster to change and not as much to mess upp. If its not your base mesh plz post it easier to see the edgeflow then.
The main point of hard-surface modelling is to get high detail, rounded edges 'n such, bake a normal map form those details, and then use the lowpoly in game. Don't worry about the tri count until you have made that lowpoly. And yes, source can use normal maps, so you are on the right track.