Alkali metal halides (AMHs) interaction with metal oxide precursors has been demonstrated to be an exciting and reliable method in the deposition of several atomically thin layered materials since the reaction rate is considerably enhanced. Using three different AMHs (KBr, NaCl, and KCl), we assisted the synthesis of MoSe2 monolayers (MLs) in the atmospheric pressure chemical vapor deposition (APCVD) process in demonstrating that it is possible to improve the crystallinity of the MoSe2 flakes and to induce the formation of 1T domains in the 2H MoSe2 lattice that can be used to perform ultrahigh sensitive surface-enhanced Raman spectroscopy (SERS) detection. We observed that even though NaCl promotes the growth of large-sized MoSe2 MLs, we found critical photoluminescence (PL) quenching originated from forming a Na-O layer underneath the MoSe2 crystals. On the other hand, resonant Raman measurements show a prominent peak around 255 cm−1 only in KBr-MoSe2 single crystals originated from a double-resonant Raman process involving the ZA acoustic phonon and the C exciton, indicating optoelectronic-grade crystalline quality, thus explicating the higher PL emission in comparison with the other two AMH-assisted samples. KCl-MoSe2 X-ray photoelectron spectroscopy (XPS) and Raman spectra display features matching the structural properties of 1T-MoSe2 polyphase, confirming the existence of 1T domains embedded in the 2H-MoSe2 lattice. Moreover, thermogravimetric analysis (TGA) measurements show that the KCl-MoO2 mixture induces different sublimation processes compared with pure MoO2, probably producing potassium-rich eutectic compounds that could stabilize the 1T-MoSe2 domains. Finally, we demonstrate that the 1T-2H MoSe2 monolayers synthesized at 750 °C showed a significant surface enhance Raman scattering activity towards the Rhodamine 6G and Methylene blue, with a detection limit comparable with plasmonic surfaces.