Hi,
In "list" mode, the setting for individual characteristics is disabled.
That's because when you have a product with 2 characteristics, if the main setting says "list", then the variants has to be displayed as a list and you can't have one with a "list" display and another one with a "dropdown" display.
Knowing that, if you really want to do that, you'll need to add some custom coding on the product edition page to change the characteristic selection mode to list based on some condition and set the default to "dropdown" so that you can configure the selection mode in each characteristic.
So you'll need a developer to help you with that.