I was on a flight years ago in the 1980s and the controller gave the aircraft I was in a climb. I knew there was traffic in our face and that the trainee controller screwed up; I told the pilots and they verified the climb and the controller said "affirmative", he didn't use the call sign. We were solid IMC and I watched from the jumpseat as the opposite direction traffic flew vertically around 50 feet lower then us. Could I have been more adamant?,yes, but, I was just a guest (controller) and it was the 1980s. Now that was a near miss!

I once failed a boldface for writing "breaks" instead of "brakes." Some things never change...

If the airplane gets bent, and you're in the jumpseat, you'll be hung with the other two.

Maybe if one is doing something blatant like yapping away during sterile, distracting the crew or saying it was ok to cross runway XX without clearance, but everything else? I doubt it.

Obviously in a serious event you'll be "going for the ride" and will have to lawyer up, but to assume you'll be hung equally for a "Foxtrot Uniform" on an airplane you're not Type Rated on, operated according to Ops Specs you know nothing about while probably not even using a headset is probably a stretch.

I of course can accept Paypal for any other free legal advice :D

Not unless you're a company check airman, or you caused the problem.

As an ACM, you're required to be available to assist if needed (ie sober). Nowhere in SOP does it designate you as PM #2.

That is just a moronic statement.

As it was said before you will probably get hung a lot easier distracting the crew at a low altitude trying to call go around in an airplane you have no type in while sitting in the jumpseat.

So, if you're in the jumpseat and you see a vehicle drive onto the runway, you're just gonna let the pilots land on said vehicle because you aren't typed in that airframe? I'm not gonna call a go around from the jumpseat just because the crew is a dot high on glide slope, but if I think they're about to crash and I'm about to die, you better believe I'm gonna call a go around. Ideally, it's dealt with early and you could just speak up to refocus the crew. That being said, 100 feet AGL with a vehicle on the runway isn't the time to be giving out "hints." Bottom line, I'm not going to keep my mouth shut while someone kills me because I didn't feel like it was my place to speak up.

Need a little common sense in these situations.

I don't buy that for a NY second.
You're not automatically a required crew member because you're in the jumpseat. Even if you're a company employee you may not be working that type.
Is there any OpsSpec/GOM/FOM that requires you even paying attention let alone be responsible for anything?

Not on APC you don't.