Hi.
I've also had problem with this part:
"And thirdly, I often see certain clips being lower than expected frame rate than other clips."
All my clips are 60 fps and I'm exporting the video as 60 fps. However, sometimes certain clips look like as if they're below 30fps, close 20 or so. Seems to mostly happening in complicated projects. To get around this, I usually need to export the Project in small sections, then combine them again in the editor.