What’s the point of slow mode/half speed

There is a trick to disabling some of the traps in that area, you probably already found it. This disables enough of the traps that you can cross the area, but not all of them:

Light all the braziers in the adjacent room to disable some of the traps

You should be able to disarm the traps if you select only a single party member and click on them when the mechanics icons appears. I've no idea why this doesn't work in your case. Though I'm not sure if you can disable these specific traps at that point, my character wasn't able to actually disable them.

You can get your party across through the traps by moving each one individually. Just click on the portrait of one of your characters, you'll only move this one then. It's a bit annoying, but possible. There is also another way around the whole thing, so if you don't want to bother, just look for another way out of the cave.

Tasks are smaller/shorter quests that generally have 3 or fewer stages and have little to no effect on your party or on any major happenings in the game.