Nice reading.
I have posted you that u can do it much earlier without any problems as a zoro count.
However minmaxing you can get the empire earlier:
1) Look at the "abys_empire" and "abys_empire_second hier"
791 and you can easily swap the emperor ( because a current emperor is in my prison) and you have 2 heirs just in case
2) normal gameplays - i`ve put screens before but will redo:
a) "zoro" 836 - my grandson is a heir to empire i have elective monarchy so i can inherit empire if i want. "zoro 40 years" i decided not to do anything more just watch the game. On one screens you are able to see that my kinsman is en emperor with a small family of heirs all from my house
b) "zoro2" 819 - i was playing to destroy empire so you see so many independent nations. Also emperor in prison can switch to zoro anytime ( what i have done anyway. "zoro_kingdom..." is a continuation
resuming:
- it is really hard not to switch an emperor to zoro faith even when u play 1 province count all the time before 820 year.
- it can be done much earlier
- you can inherit or install yourself as the emperor before 810 and possibly before 800 if you minmax.
- you can play to either destroy totally an empire from inside or install your family members or yourself - your choice.
- you are completely safe all the time as long as you play at count level.
- you can become an emperor and grand independence for everybody in the empire

- i have done it 5 times from different starts ( all the time counts) just to be shure it is easy to do. In the worst case ( when your first character is a rubbish) you will do it with you heir. Overall when you have a random start it is shorter when you will choose an old zoro count. First 30 years are boring as hell when you only spy on ppl and you have too low intrigue to aim higher so everything what you do is a waiting for a heir.
small tips:
- start as a count and install elective monarchy.
- have as many children as you can have
- don`t go duke level as long as you don`t want to fight ( from intrigue focus you should have enough money to win any fight but still it will destabilize the empire. If you want to destroy it is ok but when to inherit it is not worth to do)
- when you have high intrigue starting character you can easily play as direct vassal. Just keep your liege in prison all the time or just kill him. Constant regency.
- always prepare 2 zoro claimants or wait for them/him to have a child.
- only males can be pressed with claims.