A large number of people in my database appear in a particular census, which is one source.  So, using the census as the source, there will be a separate citation for each individual (for birth, residence, occupation, etc. information), and each citation will have its own Volume / Page entry to match the individual place where the respective person lived.
If I have understood your suggestion correctly, collapsing all to one is not an option.
That's right. It would work if your citations were empty, but in this case, it would break them. What you would really need here, I think, is a way to add the date to the source itself, assuming that all census entries were made on the same day, and without that, I guess that most put the census date/year in the source title, which may not be very consistent with the rest of your work.

Other than this, I can't give you a solution for this version of Gramps. We have no tools to set a date or make some other bulk change to a set of filtered data, like citations here. A long term solution could come from a change that's described at

but I have no idea about release plans for this yet. Templates would allow you to add a date at the source level, but they add a challenge to the GEDCOM export and maybe some other things. And in that case, maybe asking for a Gramplet that can add a date is easier.