Is there an AI control for party members somewhere? I seem to recall seeing one, but now I can't find it.

There is an AI personality option for your non-lead main character, which can select how they answer in dual dialogues, but no party member AI otherwise.

Huh. Must be a hallucination on my part.

The problem is, I've seen comments saying the AI is improved and that party members will avoid stepping on found trap triggers. But my party-members plow through whatever is in front of them. I can break them all apart and ferry them across one-by-one, but there are some obvious problems with this approach:

1) tedium,
2) danger of splitting one's team in dangerous places,
3) errors on my part due to the on-screen avatars of my guys look pretty similar - the fighter types in particular. This leads me to try to pick up and move the wrong guy (with a potentially suicidal move for the guy that I do have selected).

Am I doing something wrong? Or do I need just to deal with it?

In combat, your characters will use the shown path regardless of damage surfaces, etc. You can hold Shift, click multiple times and release Shift to have them follow a particular path.

When not in combat your non-lead characters should avoid detected traps and damage surfaces / clouds (I don't recall if they avoid pressure plates). They will walk onto a surface if you click on it, or loot there, though. If you have the lead character walk safely through an area and followers are cutting across damage surfaces or walking onto traps, then that is not normal.

***spoilers below***

Then not normal is what is happening. In particular in that cave below the graveyard (I think I'm in the abandoned home's cellar?) the repeater fireballs are triggered by walking across those bars that string across the floor. you can walk around them, by my gang prefers not to. Getting across that room is a headache. Lucky it isn't full of zombies too.

Now that you mention it, that happened to me, as well.

If you walk somewhere with a single character, you can have the rest of the party join you by using the teleporter pyramids.

I just replayed that area and I think I know what's happening (in a general sort of way). The formation-keeping behavior is trumping the individual danger-avoidance behavior. When the scubies attempt to maintain formation while moving, and to reform the formation after a move is completed, they wander a bit, and they don't seem to worry much about what they are stepping on.