First of all, thank you for providing a portable project without being prompted to.
From what I can tell, the issue here is that while VideoPad isn't crashing due to low memory, it is silently failing due to low memory. This might not be a limitation of your computer (I see 8GB, thanks for the dxdiag as well).
You can check that this is the case by exporting a small section (e.g. the first few seconds) to a much lower resolution (say, 320x240 (4:3), or 320x180 (16:9)). If the missing videos are showing in that export, then that's almost certainly the issue.
A 64-bit build of VideoPad would resolve this, but it's not ready yet unfortunately.
A potential work-around for this (assuming you want to export at 1080p or higher) is to split it into multiple steps. Starting with only 2 of the videos, export the sequence as normal, leaving a lot of blank space on the video. Then import that video as Track 1, and put a new video on Track 2, export that, import again, and repeat for all of them.
This will lead to an overall drop in quality for the video, due to repeated encoding and decoding, but it should be minimal enough. It can also be mitigated somewhat by selecting "High Quality / Larger Filesize" for the Video Quality/Bitrate (click on the hyperlink next to File Format: [.mp4 ] on the export settings screen) for each intermediate export.
If you don't want to bother with the effort of the work-around above, you could try exporting at a lower resolution (e.g. 720p), which seemed to work for me when I tested it.