Really good points, Mark.  We harp on the fact that folks should "swim-test" or "swim" their gear at various water temps (we can have a 45 degree variation between winter and summer in the mid-Atlantic region) - how else can they know what they need at 50F vs 35F.  New concept for many.

My position on any given cold water outing is that if you're unwilling to get in the water & splash around for at least a few minutes, something is seriously wrong - maybe you don't trust your gear or you know it's not enough, or it leaks "a little", or you're scared of getting wet - whatever.  All will be revealed by a little swim.  If you balk at the prospect, you really have no business being on the water that day.

People are also told to "wear a wetsuit or drysuit" when paddling on cold water, but aren't given any specifics, and few seem to have given the concept much thought.  Lots of difference between 1mm & 4mm neo & it has to be a snug fit.  Farmer Johns leave the shoulders, arms etc. exposed.  As you noted, drysuits depend on clothing worn underneath the suit. Otherwise, it has all the utility of a shower curtain.  

Thickness = warmth - the basic concept of insulation, but many
folks will "burp" their suits until they've squashed that insulation flat.  Analagous to sleeping on the cold ground in your sub-zero sleeping bag with the portion underneath you squashed flat.  Hence various pads for underneath the bag - or you can just freeze your ass off all night & get religion the hard way.

Few seem to be aware of these important nuances.  They think buying gear is akin to waving a magic wand of protection over themselves & their outings.  I tell them it's not magic, it's just gear, and all it does is buy you time; how much you choose to purchase is up to you; just make sure it's enough.

I like your picture of folks coming ashore after 10 - 15 minutes
with a newfound appreciation of cold water vs gear.  That event is brilliant.  Nothing like it in our area, but maybe this coming winter.