Several of your questions are answered in "Vedibarta Bam" on Pirkey Avos.
1. Why is it in Aramaic?
Hillel used the Aramaic, the vernacular, in expressing this important
belief in retributive justice, so that it would be understood by the
masses. He felt it important for them to know that if for any reason
whatsoever a murderer or evil-doer is not brought to justice, he may
be certain of receiving his just punishment from Hashem, for He will
not permit evil deeds to go unpunished. - pg. 86
3. Who is this skull and how does Hillel know it was drowned due to drowning others? (I have seen many citations that say it was the skull of Pharoah and source the Arizal, which I have been unable to locate)
The Arizal is in Shaar Mamarey Chazal on the Mishna.
An alternative explanation of whose skull it was:
Hillel lived in the period of history when Herod was
the King in Israel and his wife was Miriam, a scion of the Hasmonean
family. The Kehunah — Priesthood — was the domain of the Hasmoneans,
and the Malchut — kingship — belonged to the descendants of David.
Hillel was upset with the Hasmoneans for acquiring kingship, which was
not their domain.
Herod denied Chananeil the position of Kohen Gadol and appointed his
brother-in-law Aristablus, who was a Hasmonean, instead. Despite his
young age of only seventeen, he impressed everyone with his superb
performance and became highly acclaimed. This aroused the jealousy of
Herod, and he planned a way to be rid of him.
Once Herod scheduled a celebration in Yericho, and he invited his
wife, mother-in-law, and brother-in-law to participate. Present also
were Hillel the Nasi, and Shammai the Chief Justice. After the King
and Aristablus played together and perspired, the King invited his
brother-in-law for a swim. The waters were deep and very swift, and
the King secretly instructed his servants to drown Aristablus. While
in the water, they engaged in horse-play, and then they kept him under
the water till he expired. They exited the water pretending that they
knew nothing about the whereabouts of Aristablus. Suddenly, his skull
floated on the waters and everyone realized what happened and blamed
the King for it, but were unable to do him anything.
Upon seeing the skull, Hillel declared that there is an ultimate
accounting for all that one does: "Because your family, the
Hasmoneans, pushed aside the family of David from kingship, they
received their punishment in the form of the drowning of their
descendant, and ultimately all those who had a hand in your drowning
will drown." In the end, Herod gave Aristablus a royal funeral in an
attempt to remove any suspicion from himself and killed the servants
who had a hand in his drowning. (Knesses Yisroel) - pg. 85
5. Why add the word "v'sof"?
Hillel is emphasizing that nothing in this world is accidental. There
is a reason for everything that occurs. Moreover, when carefully
analyzed, one will see that it is midah keneged midah — measure for
measure. A difficulty with this theory, to some, is that at times it
appears that the no punishment was meted out or that it is not
commensurate with the iniquity. In reply, Hillel says that since we
Jews believe in gilgulim — reincarnation — it is clear that even when
the immediate punishment is not exact, however, "vesof" — "ultimately"
— when one will return to earth through reincarnation, he will receive
precisely whatever was due to him in a previous lifetime that he did
not receive then. (Midrash Shmuel) - pg. 87