Cracking is an adjective that means great ("a cracking good book"), but that doesn't make sense here, given the second sentence.
More likely it's simply the present participle of crack:
2 : to break, split, or snap apart
// The statue cracked when she dropped it.
Figuratively, the passage would mean something like this:
The sense here is agreeing with the speaker that the action will have that consequence. Namely, realising you can't sing plus stopping believing in God will have the effect of making you want to stop singing in the church choir.
It means that having lost their free time, they have gained an amazing trove of stories that they can regale other people with for the rest of their lives.
To make up for something in something else means the lack of something is compensated for by the something else.
Since the adverb "partly" is used here, we know that there are other factors affecting the subject. They are implicitly acknowledged but not stated. Hence the phrase "contribute to" would be a better choice than "account for".
Using "account for" that would imply that the listed items fully explain the situation.
You could rewrite this many ways, but one example would be:
Ctesiphon, indeed, was not worth to Persia what it had been worth to Parthia.
The construction "is ... to ..." can describe worth, meaning or relationship. For example:
"He is a friend to me".
"It is nothing to him".
In your example, the specific relation is not ...