This ballad seems to have suffered some "dings and scratches" over the years. This is how I would change it to sing for friends:

Lady Margaret she lay on her fine feather bed,
The midnight hour drew near,
When a ghostly form in her room did tread,
And to her it did appear, appear,
And to her it did appear.

"Are you my father, the king?" she said,
"Are you my brother John?
Or are you my true love William," she said,
Coming home from Scotland along, along,
Coming home from Scotland along?"

"I'm not your father, the king," he said,
"Nor am I your brother John,
But I am your sweetheart William," he said,
Coming home from Scotland along, along,
Coming home from Scotland along."

"Oh Margaret, oh Lady Margaret," said he,
"For love or charity,
Will you give me back the plighted troth
That once, love, I gave thee, gave thee,
That once, love, I gave thee?"

"I'll not give you back your plighted troth
Or any such a thing,
Until you take me to my father's hall
Where ofttimes we have been, have been,
Where ofttimes we have been."

And he took her then to her own father's hall,
And as they entered in
The gates flew open of their own free will
For to let Lady Margaret in, in,
For to let Lady Margaret in.

"Oh Margaret, oh Lady Margaret," said he,
"For love or charity,
Will you give me back the treasure troth
That once, love, I gave thee, gave thee,
That once, love, I gave thee?"

"I'll not give you back your treasure troth
Or any such a thing,
Until you take me to yon high churchyard
And marry me with a ring, a ring,
And marry me with a ring."

He took her then to the high church hill,
And as they entered in
The gates flew open of their own sweet will
For to let young William in, in,
For to let young William in.

"Oh Margaret, oh Lady Margaret," he said,
"For love or charity,
Will you give me back the plighted troth
That once, love, I gave thee, gave thee,
That once, love, I gave thee?"

Then out of her pocket she drew a cross
And she laid it on his breast,
Saying, "Here is back your plighted troth,
In Heaven may your soul find rest, find rest,
In Heaven may your soul find rest."

"Oh the winds do blow and the moorcock crow
And it's nearly breaking day,
And it's time that the dead from the living must go
So now, my love, I must away, away,
So now, my love, I must away."
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It's a good story. I like troth better than trove. For one thing, a trove is a collection of precious items, not just one. For another, troth rhymes better with cross. Finally, a troth is a promise, and I can imagine people of yore extending the meaning of troth to the cross he gave her when promised to marry.

I changed it to have the castle gates open for Margaret, because she's a daughter of the house. The churchyard gates open for William because he's dead.   

Not all the rhymes are improved, but c'est la vie.

Joe, you can mark this circa 2021.