Q. When does the Marbim Besimcha begin, on the first or second Adar? Is it already an auspicious month for business ventures?

A. See question 472 in this forum. Poskim disagree if the increase in simcha, joy and favourable mazal, apply to the first month of Adar or no.
Sheilas Ya’avetz, (2: 88) Teshuva Meahava, (2: 301) Shevet Halevy (10: 105) et. al., maintain that it it begins only in the second month of Adar.
However, Chasam Sofer (C.M. 20 – on the date of the teshuva, Chashukey Chemed Megila 6b,) Kedushas Levy (Ki Tisa,) Rav Tzodok Hacohen zt’l in Likutey Ma’amorim (16,) Sfas Emes (likutim on Rosh Chodesh,)
Derech Sicha, (Miketz p. 188,) Nitey Gavriel, (Purim – Teshuvos 2) et. al., opine that it begins from the first Adar. Nitey Gavriel (ibid. 464) quoting the Munkatcher Rebbe, adds even from Tu Bishvat. Some maintain that it includes also the month of Nissan.
There is a disagreement as to the opinion of Horav Eliashiv zt”l, (Kuntres Halichos Vehanhogos quotes that there is a mitzvah, Chashukey Chemed ibid. seems to disagree,)
Horav Shlomo Miller’s Shlit’a opinion is that it would seem from the last words of the Remoh (O.H.697: 1) that it is at least a mitzvah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a