I wonder why all of these Trident cards were set to "forced 8-bit mode"? Could it be that Trident knew that their 16-bit mode design was more flakey and less compatible?

Guess they used a fail-safe setting and put too much hope in people reading the manual...

The setting doesn't directly choose between 16 and 8 bit modes, rather, it picks between 16/8 bit autodetection and a forced 8-bit mode. And considering that anything automatic is kinda hit-or-miss on ISA (especially 8-bit machines), using a fail-safe default does sound like a reasonable explanation.