Twin value tags as an esoteric taxonomy

When importing you could create a taxonomy for each unique stance key, then add the value as a term to the taxonomy for that key, then assign that term to the post.

I’d suggest something like:

1. Store the stance keys somewhere in the database (probably as options), making sure that you don’t have duplicates. Depending on how many you have, this could probably just be a single value with a serialised array.
2. On the init hook, read the list of keys with get_option(), and register a taxonomy for each one with register_taxonomy().
3. When importing, save any new keys to the database, and immediately run register_taxonomy() for them so that the taxonomy is registered for the rest of the process.
4. For the values, add them as terms to the taxonomy using wp_insert_term(), but only after checking that value doesn’t already exist with get_term_by().
5. Once the taxonomy has been registered, and the term for the value has been inserted or found, assign the term to the post using wp_set_post_terms().

If you do that then you can easily filter posts based on these values with a taxonomy query. This will be faster than attempting to filter using post meta.