Mihi est from Brythonic to Breton I: The dative subject

Middle Breton (MB) presents a singular anomaly of pronominal argument coding. Objects are accusative proclitics save in two constructions, where coding is split by person: 3rd unique enclitics ~ 1st/2nd accusative proclitics. The constructions are HAVE, from Insular Celtic mihi est, where the new coding replaces inflectional nominatives (cf. Latin mihi est ~ sunt); and imperatives, where it replaces accusative enclitics in V1 (cf. French aide-moi ~ ne m'aide pas). The evolution is traced in light of a crosslinguistic construction type that suggests its nature, noncanonical subject + 3rd nominative ~ 1st/2nd accusative object. Part I: (1) Decomposition of HAVE as dative clitic + BE from Brythonic throughout "conservative" varieties of Breton. (2) Breton-Cornish innovation of nonclitic datives for mihi est and their subjecthood. Part II: (3) Brythonic unavailibility of mesoclisis in V1 and Breton-Cornish nonagreement with nominative objects, resulting in independent > enclitic pronouns for accusative objects of imperatives and nominative objects of mihi est. (4) MB alignment of imperatives with mihi est in 3rd person restriction on nominative enclitics, and recruitment of 1st/2nd person accusative proclitics upon loss of mesoclisis. (5) Transition to accusative objects in "innovative" varieties and subject-object case interactions