Technically those are 5 "valid reasons". Realistically...if you use the word "hitherto" in a conversation...you would confuse the vast majority of people (regardless of the place/audience). Anyone who was not confused would probably consider the usage "pretentious" (at best).

I love Sherlock Holmes! Of course the "context" is very important and if you are talking about "following a new lead" - then I would definitely suggest that you should use "onto".

Option 3 is acceptable because the construction: I was unable to + VERB (present tense) is "softer" than: I could not + VERB (present tense). Option 1 is "so direct" - it would be how I would imagine Commander Data from Star Trek the Next Generation would discuss the situation

Option 2 is preferable - "Could not have" + verb (forseen) = because it addresses the situation using the conditional tense ef.edu/english-resources/english-grammar/conditional

I don't really believe that "glory" fits very well in this situation. In fact - I'm not sure there are very many ways to correctly use "glory" in modern English. Likewise...trying to fit "great admiration" or "honour" - into that place...also does not sound correct.