[old_tv_nut]

I opened the composite image in Photoshop on my wide-gamut monitor, and it looked oversaturated. Photoshop did not find a color space specification and opened it in non-color managed mode. Then I assigned sRGB profile, and the saturation reduced to normal, that is, the same as viewing in Chrome on my sRGB monitor.

Conclusion seems confirmed that the jpg files were converted and saved as sRGB when you exported them. So again, people with default monitors and browsers are seeing them the same as you.

Question: are the originals from your camera jpg or raw files?
If jpg, they are probably sRGB to start with.

To see the full gamut of colors, the input files must be raw format, your processing software has to know your monitor is P3, and your working space has to be set to wide gamut, not sRGB. Again, it's a bottleneck issue. If any step in the process is sRGB, then the image is restricted to sRGB from that point on. It's just like if you converted the image to black and white at any step, the color would be lost from that point onward.