On of my accounts, I did not buy the Raid Pass in the first week, but I did complete all the weeklies just randomly.
The next week I bought the raid pass and all of my progress for week 1 and week 2 on the weekly challenges counted. I was able to take the rewards and the points to advance my levels.
It is important to say, that I had completed the weekly challenges within in the specified timeline. I can not confirm that you can work on old weekly challenges. I can confirm that if you had completed them, you can get the 60 points per week for the 6 levels and the weekly challenge reward.
That doesn't fully answer your question, but it is the best I can do. If I had to guess, I would say any weekly challenges that you completed you could claim, but the ones that you didn't finish you couldn't complete. But that is a guess.