If we do assume Lilith is kind of the default strongest Sinner, would that mean Alastor's deal with Rosie would've been broken had Lilith returned to Hell? If it didn't, then that would also imply that Lilith isn't as strong as everyone believed her to be, same as Lucifer.
Not sure. Rosie by just existing would have broken the deal but she is NOT just a Sinner.
Alastor died in the 30s and Lilith was still The Queen until 2020's (if we go by the pilot being in 2019). So maybe Lilith was the Strongest Sinner but by marrying Lucifer she became QUEEN and therefore, a different status. Wordplay playing in favor of Rosie.