1. What I was wondering was simply to offer an alternative of "col". So you would have the choice of using fractions that add up to 1 e.g.
1/3 1/3 1/3
1/3 col 1/3
the second approach would mean that bootstrap take care of the empty column for us with no need for fancy coding
I suppose an alternative would be to use something like
1/3 1/3 1/3|1/2 1/2
So if there are 3 columns class names would be "col-4 col-4 col-4" but if there was one empty (and skipped) it would use "col-6 col-6"
That would be "fairly striaghtforward" to implement.
2. For the row based conditionality I was thinking about your custom field criteria - I would implement it like we implement the specific field output currently.