The truth you'll tell to me, Tamlane, a word.
You must not lie.
Have you ever been in a holy chapel
or blessed by Christianity?

I'm pretty sure it's "a word ye mauna lie", all one phrase. Also "sained in Christentie", not 'by'. So I'd say it's

The truth you'll tell to me, Tamlane,
You must not lie, not even a word
Were you ever in a holy chapel
or consecrated as a Christian?

My impression is that 'sained' is quite a strong word - 'sanctified' or 'made holy'. Which is what the sacrament of baptism does: it makes the baptisee holy by admitting him/her to Christianity.

The truth I'll tell to you, Janet, a word.
I will not lie.
A (human) knight fathered me and a (human) lady bore me,
just as a knight and a lady did thee.

Again, I'd go for

The truth I'll tell to you, Janet,
I will not lie, not even a word.