The details of a guaranteed evolution of Eevee into Umbreon are said to be:
Walk at least 10km with Eevee as your buddy
Earn at least 2 candy while walking
Evolve at night
These details leave a lot of room for interpretation and I'd rather not risk wasting the 25 candy. I have the following questions:
On the buddy details page 2, I have 24.02 km walked together from previous times I had Eevee as my buddy. Does this count towards the 10km, or does it have to be since I last assigned Eevee as my active buddy?
The same question mostly, since every 5km gives 1 candy, I have the 2 candy earned, but does it have to be since I last assigned Eevee as my active buddy?
What specifies as 'night'? During which hours? Between 23:59 and 6:00? And related to which time zone?
If you only want to evolve Umbreon for the Pokédex entry, the easiest way is to rename your Eevee to Tamao and it will 100% evolve into Umbreon without needing any of the other requirements. Note that this only works once. This is possible once-per Eeveelution, see this answer for all possible names that work.
Another tip is that the evolve button will actually show Umbreon, or a silhouette if you don't have it in your PokeDex, when the Eevee has satisfied all the requirements for evolution. If the evolve button shows a question mark it can only evolve into the three Gen1 options, and as soon as the walking/candy requirement is done, the Eevee will switch between showing Espeon or Umbreon in the evolution button. Which it shows will depend on if the game registers day or night conditions, but will act as an indicator to let you know the result for those specific evolutions.
To answer your questions specifically:
The 10km is counted over the lifetime of the Eevee being your buddy, so you can walk it 10km, remove it as your buddy, put it back as your buddy, and it fulfills the requirements.
Same as 1.
Night changes based on your locality. Usually the switch from day to night happens around 6-7pm, but it will change based on season. It's not exact with official sunrise/sunset times for your area, but is usually within half an hour.