Cody Robertson has suggested (5 April 2006, conveyed by Camille Fronk) that the or near the end of this verse may be an error for the preposition for (thus “to his glory for the welfare of the ancient and long dispersed covenant people of the Lord”). If such an error occurred, it probably took place during the dictation of the text since both 𝓟 and the 1830 edition are here firsthand copies of 𝓞 and each reads or, not for.
There is some evidence that or and for were occasionally mixed up in the transmission of the text, including two examples where Oliver Cowdery was responsible for the mix-up (the second and third cases listed below):
In the second example, Oliver Cowdery initially wrote for in 𝓞 (undoubtedly under the influence of the preceding for in “for the freemen had sworn”), but then he immediately corrected for to or by erasure, giving “for the freemen had sworn or covenanted to maintain their rights”. In the third example, both 𝓟 and the 1830 edition are firsthand copies of 𝓞; the 1830 edition reads for and 𝓟 reads or. Here the or in 𝓟 is probably the result of the following or: “or else he will hold to the one and despise the other”. This passage follows the King James text in Matthew 6:24, which has for here. This last example shows that Oliver could have misread an original for as or. Thus there is scribal evidence to support the possibility that Oliver wrote or in Mormon 8:15 rather than for when he took down Joseph Smith’s dictation. However, in Mormon 8:15 there is no nearby or that could have triggered the error. In fact, that passage has two preceding instances of for (“for none can have power … for God will that …”). For a case where or may be an error for for, see under Ether 8:24.
Internal evidence argues for the preposition for typically occurring before noun phrases with welfare as the head (14 out of 20 other places in the text):
The example in Helaman 12:2 is of the form “to do something for the welfare of someone”, like the proposed reading for Mormon 8:15 (“it shall be done … for the welfare of the ancient and long dispersed covenant people of the Lord”).
Another possibility to consider here in Mormon 8:15 is that the original text read or for and that the for was accidentally lost as Oliver Cowdery took down Joseph Smith’s dictation:
The loss of for following or is quite possible, given the similarity of the two words. As we might expect, there is evidence for or for elsewhere in the text:
Finally, one may consider the earliest text in Mormon 8:15 as a case of ellipsis of the preposition to —that is, the passage is equivalent to “with an eye singled to his glory or to the welfare of the ancient and long dispersed covenant people of the Lord”. Correspondingly, it is also possible that an original repeated to was accidentally omitted here during the dictation of the text. And there is evidence in the history of the text for the repeated preposition to being omitted after the conjunction or (including one example, marked below with an asterisk, where Oliver Cowdery made the error):
Ultimately, it is difficult to decide between all these possibilities. Probably the best solution here in Mormon 8:15 is to maintain the earliest reading (which is the reading of all the textual sources) since it will work if we interpret the reading as a case of ellipted to. Yet the possibility remains that the or alone is the result of some sort of primitive error that occurred as Oliver Cowdery took down Joseph Smith’s dictation, with alternative readings for the original text being for, or for, or or to.
Summary: Maintain in Mormon 8:15 the earliest text that conjoins two noun phrases by means of or, thus “it shall be done with an eye singled to his glory or the welfare of the ancient and long dispersed covenant people of the Lord”; various alternative emendations suggest themselves, but since the earliest reading will work, it will be kept.