Merging data files results in misalignment of time

I have a collection of GoPro files I want to concatenate, but there are gaps between them.

When I load them all and their associated data files, then join them, I encounter two problems:
1) Each data file is a couple of seconds longer than the video. This isn't a problem individually, but results in an growing mismatch. I can fix this by manually trimming each file to match the length of the video, but it's a PITA.

2) When I "merge" the data files, the time gaps between the files get collapsed - so when I have a timer showing time of day, rather than it reflecting the 5 minute jumps between runs, it gets misaligned with the video (I can tell, because I have a time overlay on the GoPro). This only happens when you use the "merge" option, and I've verified it by comparing the merged vs. individual CSV exports.