Make full use of whatever awesome multifile read/edit the software presumably has already and then maybe go a little beyond. The "configuration" csv might have some hashbang equivalent defining a line offset for the header that isn't mapped into input/output files (perhaps a concept of "negative line numbers" for project properties?), contains references to those peer files and maybe activates project options like "display peer file contents in cells that are otherwise empty" if you want your working surface to remain "2d".
The argument to have this add-on (I'm not sure that it should be part of the product or a separate extension) would be that it would be a shame to learn all the UI ergonomics of the plain reader/editor and then not be able to leverage them for some calculations as well.
I would actually be wary of trying to make this too much like a spreadsheet, even in appearance. You'll be pulled into that strange attractor, but you can't hope to compete in that space. Whatever oxygen isn't consumed by Excel itself is long gone between LibreOffice and online spreadsheets.
Another possibility would be integrating with SQLite via CSV export and giving people an SQLite console: https://www.sqlitetutorial.net/sqlite-import-csv/