This paper provides fresh insights on how PIs (Polarity Items) in non-veridical contexts (questions and conditionals) are represented in the grammar of multilingual learners of Catalan at different stages of development. It explores how this non-native grammatical system interacts with other previously acquired systems of negation and the implicit and explicit evidence about the negative system as presented in textbooks. We test 89 Ln learners of Catalan (L1 English/Italian/Spanish/Portuguese) on the interpretation of PIs in non-veridical contexts. The findings reveal that initially, learners exhibit both facilitative and non-facilitative transfer from their prior languages (L1/L2), and that grammatical restructuring in L3/Ln development is influenced by learners' language background, syntactic context, and proficiency in Catalan. The results indicate a tendency towards stagnation and reduced accuracy throughout the learning process, especially among learners whose L1 had a facilitative effect either during initial transfer or later stages of development. These observations are discussed in the context of theories of morphosyntactic transfer in L3/Ln acquisition, as well as L3 development. The paper also evidences that the presentation of PIs in textbooks can shape and restructure learners' interpretation of these items and proposes acquisitionally and pedagogically informed criteria for the instruction of PIs to Catalan learners.