1) Some rather scary bit of information became known to him concerning something on the level of what the Chinese might be doing militarily.
2) He just wanted a pastor and to pray because he has to issue some orders he's known for some time he had to issue and he wanted to ask God that the people who must carry them out be safe in doing so.
3) I tend to think the part about "something that changes everything" is just to make it seem as if information came to light in the last 24 hours that it going to explain the actions he is going to take in the next 48. The only thing that has really changed in the last 24 hours and in the next 24 is that what has been known for a bit now to Trump's team is going to be known to many.
GP is not the source.
Here’s the source with DNI Ratcliffe’s letters included:
https://www.washingtonexaminer.com/news/intelligence-analysts-downplayed-election-interference-trump-inspector.
You are spot on in your 2) and 3). You’re off on 1).
If you read the link above and look at the dates on Ratcliffe’s letters, you will see that POTUS has known of this for 10+ days. 10+ because the letters are formal records. The information came before in direct conversation.
Look at the timeline where the Military units reporting direct to POTUS were dispatched. That timeline of troop movements matches up with Ratcliffe’s report to the President.
The pastor is very likely genuine to the report as Trump has become a devout Christian and knows he is up against horrible evils. He is leading the United States and its People into a moment of history that will be remembered beyond the lifetime of anyone living today.