The 1849 LDS edition introduced a switch in the word order here in Mosiah 7:23. This change, apparently a typo, has continued in all subsequent LDS editions. We expect (even in modern-day English) the inverted word order in exclamatory statements beginning with a question word like how, as in the following sampling from the text:
But the text also has a couple of exclamatory cases where how is followed by the noninverted order rather than the expected inverted one:
This variability suggests that in each case of this type we should follow the earliest textual sources. Thus in Mosiah 7:23 the original inverted order have we should be restored.
Summary: Restore the expected inverted order have we in the rhetorical exclamation in Mosiah 7:23 (“how great reason have we to mourn”).