If you pull each raw submission then you can choose to process them however you want.
Not for any time soon. What is your scenario?
Typically when I've removed fields it's been because they turned out not to be actionable and there's no value to keeping them around. One case I can think of where I have wanted to retain the data for a column is when I used a "Is this practice or real data?" question. I wanted to remove it once data collectors were not practicing anymore and all data was real but I still wanted that column in my data to be able to filter out practice submissions marked before I removed the question. I set the relevant column to false() to achieve that. That way, the field is still part of the schema meaning it is included in the export. Maybe what we can do as a first step is to document this strategy.