This is something I've been working on lately Tess - there are a couple of ways I've done this. I pretty much never record the narration while I'm recording the screen - although as a technology trainer I should be able to do this, but I find it a very different situation when you know you're recording something as opposed to showing learners in a face-to-face training session.
So, I firstly go through the process a few times before I start recording. Then I do the screen recording using SnagIt (which I use a lot and find an absolutely brilliant program). Then I bring this .mp4 file into Camtasia where I edit it (sometimes quite heavily). I then record the audio while I'm watching the edited file - this is really easy to do in Camtasia. This way it's easy to synch the audio with what's on the screen - or work with both a little to get this right after I've finished the recording the audio. Then I publish the Camtasia file into a new .mp4 and import this into Storyline.
I wish I could do all of this in Storyline, but the draw-back with doing this whole process in Storyline is that even though you can record audio directly from a slide in Storyline, you can't watch the video as you're recording the audio.
I really like your concept of using some static images amongst your video footage - I'm going to try this myself. I have also done some screen recordings with narration where I have added pop-up messages, such as relevant keyboard shortcuts or additional tips on things that I may not have narrated in the audio. Too many of these can be quite off-putting, but sometimes these can be very useful and add to the visual engagement. I have done this in both Camtasia while I'm editing the video and audio, and also in Storyline by adding these to the timeline at the relevant points along the video.
As far as being a seamless process, I'm not sure that one way is any better than another - I find that the more I stick to one method, the more familiar I become with the process and then it becomes more seamless.