Regarding the questions above, here's my best attempt:
1. There is no source as to when Patchouli arrived at the SDM. However, she has spent about 100 years (i.e. all her life) around books, and PMiSS states that since the library is very large, it is likely she has lived there for a long time.
2. There is no source as to when Meiling arrived at the SDM, and no evidence to support any date at all.
3. The SDM has been confirmed to be in Gensokyo as early as season 113 (1998); it can be assumed that Remilia and company were residing in it at that time, though this cannot be ascertained. If Patchouli was residing in the SDM at the time of the jump (likely, given that the library contains a large number of books from the outside world) then it must have occurred less than 100 years ago. According to the timeline on thwiki.info, Akyu believes that Remilia arrived after the Vampire Incident, however I have yet to find the evidence that supports this. Combining these, the most likely period for the SDM's arrival in my opinion is shortly before 1998, though of course this is only speculation.
4. The first vampire incident is explicitly stated to occur after Gensokyo's sealing, and assuming the above, must also have occurred before 1998. Any dating in between these is unclear, though it is likely that it occurred relatively close to the implementation of the spell card rules (EoSD, season 118 or 2003 CE).