Not a big deal on the mold line, you can take care of it and go back and touch up the model. I know from having done it a lot
If you have a bolter on this guy and the rules don't allow it, just say it counts as a bolt pistol. It isn't that far away from a bolt pistol, if you want a different look, then do it. I mean there's a big difference from saying "Oh yeah, he's holding a bolter but I want it to count as a bolt pistol in the rules because I wanted him to look a little different" than oh I have some female Space Marines with Eldar weapons that use Ork rules.
I have a second edition Khorne berzerker that is holding a bolter.
You can mix a darker red and blend that in a little and then highlight that with lighter red over the top of that. The wash isn't TOO much on this from the pics if you wanted a dirty look.
One thing more that I would suggest is to drill out the gun barrel.
Anyway, good work!