1.A) the game ends ends in 27 B.C. and the first triumvirate was in 60 B.C.(that's about 30 years not exactly pivotal).
B) yes there we triumvirates before this but it was a government position and from what i can read,and it looks like they were assistants to the high judge not a major political force in Rome
C)so the first important triumvirate was just an alliance with some of the most powerful people in Rome so i don't know how this will be shown? would it be the heads of the families uniting for a civil war or so you could have increased loyalty for some characters????
2. obviously if its alt history any thing could happen but that doesn't mean we need a random mt. Vesuvius erupts in Rome event.
3. maybe but even so it will probably be in a while and this feature may come out with the imperial expansion or what ever they call it.
4.you don't role play a character in I:R you play as the state (which is why they don't have the cursus horum and can imprison your leader) so i'm not sure about you but having some random people form an alliance that you can't control doesn't sound to fun.
need need need need need need need need need need need need need need need need need need need need need need need need need need need sources:
https://www.britannica.com/topic/triumvirate
https://imperator.paradoxwikis.com/Imperator_Rome
https://www.ancient.eu/Vigiles/