For the first group, only “There is every reason to celebrate” sounds natural. The versions with “for which” and “for” do not work here. For the second group, all three are possible, but “She found a quiet spot to read” is by far the most natural. “At which to read” is very formal, and the final “at” is unnecessary. For the third group, “We shall have many days to talk things over” is the most natural. “On which to talk things over” and “in which to talk things over” are also correct but formal. The other versions are either unnatural or change the meaning. The main point is that English often uses a noun directly followed by an infinitive, as in “reason to celebrate,” “spot to read,” and “days to talk.” I feel it's also important to mention that native speakers from the US don't use 'shall' in everyday speech unless they are purposely trying to speak very formally. This sounds more literary and even more poetic. So if it's meant for formal writing, that's fine, but it sounds a bit too formal for everyday speech. Instead, I would use the word 'will'.

1a 2a

Hi 1a. There is every reason to celebrate. = There are many/good reasons why we should celebrate. 2s are all correct and they all mean “She found a quiet spot where she could read.” In 3, the most natural ones are: 3a. We shall have many days to talk things over. 3b. We shall have many days on which to talk things over. 3f. We shall have many days in which to talk things over.