Well, you ask a lot of question in one, but let's see how far I get :)
- 1) Upon refers to a period. One of the things the solution would result in is a period, and (the length of) that period is something that needs to be agreed upon. In other words, no period is decide yet, but a decision should be made.
- 2) No. The sentence talks about "a comprehensive solution". That solution is builds on initial measures. That solution will then result in a final step that consists of two parts: a period of time during which this solution will be in place (that is the period to be agreed upon), and a resolution of concerns. A resolution of concerns means that current concerns (worries) will have to be taken away (resolved).
- 3) There is no past tense, actually. I assume you think that would build is past tense, but it is actually future: the would indicates a (likely) possibility in the future (if and when a final solution will be reached). Compare to "I'm thinking about buying a car. It would be a red one with four wheels." This doesn't mean I bought a car in the past or that it was red in the past. It means that if and when I buy a car, I expect it to be a red one. And I expect it to have four wheels.
- 4) I am not sure how or why you want to use of which, but you could write "This comprehensive solution would build on these initial measures and result in a final step for a period of which the length needs to be decided and the resolution of concerns."
- 5) I took the liberty of editing your question, and I fixed some wordings and turns-of-phrase. If you look in the edit history, I think your question on this is answered.