is technically a vocative, which in English appears in the nominative (subject form)

"Ye" isn't the object, it's the subject form - "you" is the object:
Ye see me - I see you.
Hence it should be "God rest you merry, gentlemen"
but nobody ever sings / prints it that way... *sigh*

In Sweden, which probably explains a lot...
Not long after I left Sweden, I heard that my church was running a kind of "inner peace" services aimed at people seeking for spirituality outside of church, which included communion as a spiritual experience, regardless of any faith in Jesus... 😶

True; I just haven't heard it spoken of in those terms. I also don't recall it ever being discussed when I was training for ministry, which in hindsight is a bit odd...

TBH I haven't heard, or used, any of those phrases!
I just say something along these lines:
Taking Communion means admitting I'm a sinner in need of a Saviour, and Jesus is the Saviour I need.
If that is true of you, you're welcome to join in; but if you're not sure about Jesus, you'd better not.

That's what my wife would prefer I did, too... 😄 Whereas I like to check every now and then: in many cases I prefer to fire off a quick reply, either answering the question or letting them know I'll deal with it when I'm back, rather than have things build up...

I also have two phones, one personal and one ministry - although I don't separate them as well as you seem to, and I usually take the ministry one with me on holiday - although it stays switched off most of the time.

And also praticality: unless tall books lean against the side of the bookcase they will fall over...

Yes, organising by colour betrays a serious disregard for what books are...
Having said that, we have several bookcases and they are organised in different ways - the linguistics one is based primarily on height...

But... the glory of English is that you can verb anything!