I don't know if anyone'll know this, but are the more advanced stances (lower, middle, upper, vitals) equivalent to stancing all the related body parts at once, or are they a equivalent to a less effective stance with wider cover?

It's the latter. Stancing more than one limb will make you unable to dodge as effective as stancing a single limb, the same as with parrying. The major benefit of Combat is at Trans if you're stancing the right limb you'll be near untouchable by warriors.

Wonder, should I trans combat or resilience first?

Combat increases wounding/damage too.

Not much though I wouldn't think...

correct

If you stance lower i'm going to be able to hit your legs easier than if you just stance legs. I hit maybe 1 out of 8 hits on someone with trans combat, especially if they have decent dex.

After expermenting (I have an unhealthy obsession with developing parry/stance systems) Its better to stance the individual area rather than the broader stances.