(1) The RASHBAM himself actually explains that Rabah was not eating before Shabbos entered. Rather, he was of the opinion that Kidush should be recited before the table (or Chalos) is brought out ( after Shabbos entered). However, the members of the house of the Reish Galusa brought the Chalos out (again, after the beginning of Shabbos) before he recited Kidush, and therefore Rabah covered them.

The Rashbam does cite an alternative explanation in the end of his comments, which says that they were eating before Shabbos entered. He refutes that explanation though for a different reason other than that which you asked.

However, the second explanation is that of RASHI here, who clearly says that they were eating before Shabbos entered. It must be that they started when it was permitted to eat on Erev Shabbos (see Insights to 100a), and did not intend to go on eating for so long, but it happened that their meal went on until nightfall. That is also what we must say regarding the Talmidim mentioned later (105a) who were eating a meal when Shabbos entered.

(2) Good point. We also asked your question here in the Kollel, but we did not find anyone who discusses it. It could be that this was not referring to the day of the Chupah or to one of the seven days of Sheva Berachos after it, but it was within thirty days of the Chupah, during which there is still a degree of celebration, but there is no obligation to have a festive meal.

Alternatively, perhaps Rav Huna was an Avel and could not attend the Sheva Berachos of his son, except when it was necessary to do so in order to eat on Shabbos (that is, when his light went out).

Best wishes!