cap 21: let us pray

Yes, he knew and was playing along.

It is difficult to say exactly which scenes/dialogue was changed in making Georg a stronger character without having seen the original script, because so very much was changed from the stage version. The graveyard scene is the standout, as that is similar to the stage script.

reverendcaptain always asks the best questions!
I would dare to say that he would always let her lead the prayer, basically because of Maria's religious role (which makes everything quite logical) and because doing it himself (especially if they coincide at the table with Elsa or Max) would be like facing Elsa in a figurative way (for me, a little strange).

(btw, my grandparents used to pray at the table and although my grandfather clearly led the table, she was always the prayer)

You never know but I always imagined him as a believer. It is assumed that the Captain had a certain relationship with the convent since the RM shows great respect for him from the beginning and it is assumed that he goes to the abbey looking for a governess (or I am already influenced by the fanfics?) or at least there is a hint of camaraderie between the Captain and the RM.
There is also a crucifix in the governesses' room (it can be seen while Maria is praying, although giving a second thought, it makes more sense that she herself would have carried it with her...)
However, at the dinner time, he is clearly upset but the children do not hesitate for a single moment how to behave while she is praying which indicates that they probably prayed in the past.
Also, would Maria have fallen in love with him so deeply and easily if they hadn't shared the same faith? ok everything is possible when it's about love but let's be honest, she was a postulant.

I don't know if there is an specific thread but now that I think about it, the RM-Captain relationship is curious.

She begins by describing him when she entrusts the mission to Maria:

"I will tell Captain von Trapp to expect you tomorrow (...) A retired officer of the Imperial Navy. A fine man and a brave one." They have clearly talked directly and not through third parties, perhaps even have a long-standing friendship.

He then refers to rhe RM again in front of Maria: "Now, fräulein... -Maria. I don't know how much the Mother Abbess told you..."

Later and after the kiss! they both talk about the Mother Abbess: "What else does the Reverend Mother say?" I mean, by God, they were kissing and they start talking about the RM??! without a doubt a great friend for both of them (I even wonder if Georg wouldn't be so nervous minutes before asking Maria her reasons for coming back fearing that she had told secrets to the RM hahaha).

And then, in the middle of the escape, the Captain turns again to the RM like a son in search of his mother's help: "We thought we might borrow your car", it is definitely a closer relationship so my vote is that it was believer and a practicing Christian.

(I know that in real life you can have friends and not share the same ideas, but this would not be so much fun to argue).

Asking about her safety wasn't a good option because Maria already wrote in her note where she was going and asking if she's there already might sound a bit ridiculous? especially if you're calling the abbey dressed in a tuxedo 😂😂 Or is the day after the party with the hungover when he recalls? I'm sure we've already talked here about when Georg reads the note.

Regarding your question: "But it still seems strange to me that a married sailor with 7 children knows the RM so well" I can think of many good reasons, from knowing each other as children to professional relationships (donations from a wealthy family to a convent for charity or the passage of other governesses in the past), or simply because it turned out to be a family with faith that they frequented the church or maybe went to mass at the convent when they celebrate.
In any case I love your theory about the Captain's stuttering call! but something tells me he's so embarrassed after being discovered by Elsa in the Laender that he might not have called directly, even though he was dying to. Maybe that's why he's so desperate afterward to wring an explanation out of the children and their blueberries 😂

I think this is spot on (love it!) but my two cents to add is that he has 'fallen off the wagon' after the death of his wife. It would seem easy to become cynical - praying to God didn't do him much good when his wife was ill. I suspect he was very devout (with his wife), put many a donation in the collection plate over the years...and then went cold. I think the RM would understand this kind of relationship with the church given Georg's circumstances. Georg just looks so darned unimpressed with all aspects of grace at the dinner table.