Although you're right, that is exactly how it was published in the Manchester Hymnal, which is the text being quoted, as it was published. (I checked to be sure.) I'm not long to misrepresent the Manchester Hymnal by changing that (there would be little point in listing the text as it is from each source, otherwise). However, I can make a note of this so that people know it was an unintentional, and is, at least in both of our opinions, better represented with an exclamation mark.
If the question mark was purposeful, I think they intended it to inspire reflection as to what all that comfort might be, much as in counting your blessings—not necessarily to question it (kind of like a rhetorical question).
I think it's just a typo - a bit like 'knoiw'! I was really checking to see what the original was, as our old hymn book had 'improved' it by removing a lot of the "He lives". It does make it read better as a hymn, but also loses something as a result. Another book rather overdoes the exclamation mark by using it at the end of every line!
Thanks for the prompt response.