1. The Mitzvah is too central to be counted individually.
2. His computer had a virus.
3. After the Exile the Mitzvah ceased to be obligatory.

Correct. Living in Israel adds a new dimension to the observance of every command (Nachmanides). Most of the commandments are only applicable when the majority of the Jewish people are in Israel, and they can only be a "light unto the nations" when living in Israel according to the Torah (Kuzari). Due to the universal nature of this Mitzvah it cannot be counted as an individual commandment (R.Kook).

1. The Mitzvah is too central to be counted individually.
2. His computer had a virus.
3. After the Exile the Mitzvah ceased to be obligatory.

Maimonides' computer NEVER had a virus!

1. The Mitzvah is too central to be counted individually.
2. His computer had a virus.
3. After the Exile the Mitzvah ceased to be obligatory.

Correct. Some commentaries maintain that after the Babylonian Exile the Mitzvah was no longer applicable, as the commandment was limited to the original conquest of the land under Joshua, the prophets and King David.