Q. Since we require that the seven weeks counted before Shavuot should be temimot and complete, we wait until night to say Maariv. Why is that some women light candles early and even say shecheyanu then?
If a woman that has the minhag to light early and then say shecheyanu married a husband that is a baal teshuva and does not have any family minhag but belongs to a shul where most people traditionally light later, what should she do?

A. On question 291 we wrote; Piskei Teshuvos (O.H. 494, 4) rules that women should wait to light candles on the eve of Shavuos until nightfall since they recite the brocho of shecheyanu which is similar to kiddush and they accept at that time the sanctity of the day. The fact that no sefirah was counted is of no consequence; you still have to keep the seven complete weeks. A man that missed counting would also wait with Kiddush until night.
Horav Shlomo Miller Shlit”a disagrees in principle and mentions that the common custom is for women to light candles earlier as in every other Yom Tov (as it is recorded on most calendars). Men should also observe and accept toseffes Yom Tov before sunset. This does not contradict the compliance of keeping the seven complete weeks, as the total Kedushas Yom Tov is received only when reciting Kiddush at night.
On question 559 we wrote; Mishna Berura (494: 1) writes that we wait to recite Maariv until nightfall so the forty-nine days of the Sefirah are completed. Although Sh’loh (beginning of Shavuos tractate,) Mogen Avrohom (ibid.) and others maintain that we are only concerned with making kidush and eating before nightfall and not with reciting Maariv, many actually do wait. As far as women lighting candles before or after nightfall, there are different traditions. Lehoros Nossan (7: 31) explains that even though women are exempt from the time linked positive commandment of Sefiras Haomer, waiting until nightfall in Shavuos is not a direct result of complying with this mitzvah. Rather it relates to the fact that the date of Shavuos is not mentioned in the Torah but depends on the omer counting, therefore to follow that dictum, we should light candles after completing the count. Horav Shlomo Miller’s Shlit’a opinion is that one should follow his family tradition. When that is not known, he suggests to light early as with all other Yomim Tovim. (He quoted Shulachan Aruch Horav that mentions; to have in mind receiving only Tosafos Yom Tov and not the kedusha of Yom Tov Itself).
In regards to your second question, Horav Shlomo Miller’s opinion is that although normally the wife follows her spouse’s minhagim, in this case since the husband lacks a known prior family tradition he should follow his wife established tradition. In addition, it could well be that generations back, the husband’s family kept the same tradition as the wife since it is more common, as mentioned above. Besides, in regards to the tradition of the husband’s shul, you wrote in your question; “most light later,” apparently some also follow his wife minhag.

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