No problem with "heaven" if it's pronounced "heav'n".
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I tried looking up Oh Little Town of Bethleham in a Moody and Sankey collection printed back in the 1890s, and found that the verse I mentioned was missed out then - but the Oxford Book of Carols (edited by Ralph Vaughan Williams) has always included it. Incidentally the Oxford Book of Carols mentions that the tune is "The Ploughboy's Dream - and a Google search came up with the words for that, and here they are, and very singable I'd say. And shows a very encouraging concern for animal welfare as well:

THE PLOUGHBOY'S DREAM

I am a ploughboy stout and strong as ever drove a team
And three years since as I lay a-bed I had a dreadful dream
I dreamt I drove my master's team three horses travelled far
Before a stiff and armoured plough as all my masters are.

I found the ground was baked so hard 'twas more like bricks than clay
I could not cut my furrow through nor would my beasts obey
The more I whipped and slashed and swore the less my horses tried
Dobbin lay down and Belle and Star ignored my threats and cries.

Till low above me appeared a youth he seemed to hang in air
And all around a dazzling light which made my eyes to stare
"Give over cruel wretch" he cried "do not thy beasts abuse
Think if the ground was not so hard they would their work refuse".

"Besides I heard thee curse and swear as if dumb beasts could know
Just what your oaths and cursing meant it's better far than gold
That you should know that there is one who knows thy sins full well
And what shall be thy after doom another shall thee tell."

No more he said but light as air he vanished from my sight
And with him went the sun's bright beams 'twas all as dark as night
The thunder roared from underground the earth it seemed to gape
Blue flames broke forth and in those flames appeared an awful shape.

"I soon shall call thee mine" he cried with a voice so clear and deep
And quivering like an Aspen leaf I woke out of my sleep
So ponder well you ploughboys all this dream that I have told
And if the work goes hard with you its worth your wage in gold.

I've never come across this - but I here's a YouTube clip of a rather strange arrangement from 1974 by a band called Gryphon

Pull on a loose thread and youn never know where you might end up...