Avos kept the Torah in the strict sense that OP defines including takanos derabonnan only when they were inside Eretz Yisrael. Outside of Eretz Israel, since they were not commanded to observe the mitzvos they could violate them in certain cases in extenuating circumstances. For example,
- Yaakov married two sisters in Lavan's house. He had to marry Rachel, but Lavan forced upon him marriage with Leah. When the family returned home to Eretz Israel, one of his wives, Rachel, died as discussed here.
- Abraham gave his guests dairy food as a snack prior to them having a meat meal can be seen in the same way as we should ideally have a Shavuoth dairy meal as described here.
- One can even say, that arguments between brothers parallel the arguments Bais Shammai and Bais Hillel. Mishna Yebamos 6:6 says:
A man may not neglect the mitzva to be fruitful and multiply unless he already has children. Beit Shammai say: One fulfills this mitzva with two males, and Beit Hillel say: A male and a female, as it is stated: “Male and female He created them” (Genesis 5:2).
- Yosef son of Yaakov had 2 sons and stopped having more children due to upcoming famine since he fulfilled the mitzvah according to Beis Shammai. Levi son of Yaakov continued having children until he had a daughter Yocheved according to Beis Hillel.
Amram and Yocheved lived in Mitzraim, outside of Eretz Israel. Remarriage of Amram to Yocheved is described in Gemara Sota 12a when she was 130 years old. The gemara points out that there was a miracle:
Rabbi Yehuda says: The signs of a young woman were born in her when her husband remarried her, and she became like a young girl again.
The same story is also discussed in Midrash Shemos Rabba 1:19. Interestingly, Midrash Sefer haYashar, Parshas Shemos 19 uses the same language to the original marriage of Amram and Yocheved:
איש היה בארץ מצרים מזרע לוי, ושמו עמרם בן קהת בן לוי בן ישראל. וילך האיש ההוא ויקח אישה את יוכבד בת לוי אחות אביו, והיא בת מאה ושש ועשרים שנים ויבוא אליה. ותהר האישה ותלד בת ותקרא שמה מרים, כי בימים ההם מיררו מצרים את חיי בני ישראל. ותהר עוד ותלד בן ותקרא שמו אהרון, כי בימי הרותה החל פרעה לשפוך דמי זכורי בני ישראל.
And there was a man in the land of Egypt, from the seed of Levi, and his name was Amram, son of Kehath, son of Levi, the son of Israel. And this man went and took for a wife Yokheved, the daughter of Levi, the sister of his father, and she was one hundred and twenty-six years of age when he came unto her. And the woman conceived and bare a daughter, and she called her name Miriam, for in those days the Egyptians embittered the lives of the children of Israel; and she conceived again. And bare a son, and she called his name Aaron, for in the days of her pregnancy Pharaoh began to spill the blood of the male children in Israel.
The midrash proves that the original marriage of Amram and Yocheved happened under extenuating circumstances. As Amram was Gadol haDor the miracle with his 126 year old wife (R. Chaim Kanievsky corrects 124 year old) served to show that even in such circumstances people should continue having children.
Family is built on mutual trust. All Avos married within a family, for example, because it is easier to have trust to one's own kin as Avraham instructed Eliezer. Eisav was the first one who married outside the family. However, his most expensive item he kept in his parents' house. It was the coat that he took from Nimrod, which Yaakov borrowed to get a blessing from Yitzchak. Eisav did so in relatively good times because he could not trust his wives. Many Rabbis in gemara married their nieces for this reason. Difficult times put big strain on relationships that are not strong. Amram was gadol hador, and did not want to endanger shalom bayis due to extenuating circumstances of crushing Egyptian hostility.
