On the day of Shravana Dwadasi: Can those who do not observe Shravana vratam, do Dwadasi paranai? Or if paranai is allowed only on Trayodasi, can vrata anushtanam consider Ekadasi as secondary and consider Dwadasi as the primary day?

Shravana Dwadasi and Shravana vratam are not related. Shravana vratam is a kaamya vratam, undertaken with aim of achieving an objective. It can be for a particular want or Emperuman but fasting is not a must.
Shravana Dwadasi is called ‘Nityam’; that is, it is a must to fast on that day.
One must fast both on Ekadasi and Shravana Dwadasi days and paranai (breaking fast) is done only on Trayodasi. If physical conditions are a limitation, one can partake in a limited manner on Ekadasi day and fast on Shravana Dwadasi day. It is the practice of the elders.