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: 3^rd unique enclitics ~ 1^st/2^nd 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 + 3^rd nominative ~ 1^st/2^nd 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 3^rd person restriction on nominative enclitics, and recruitment of 1^st/2^nd person accusative proclitics upon loss of mesoclisis. (5) Transition to accusative objects in "innovative" varieties and subject-object case interactions.