Here the earliest text has a singular subject and verb, but the subject complement is in the plural (“it is the everlasting decrees of God”). Joseph Smith removed the disagreement in number by changing the plural decrees to the singular decree. One could argue that the plural decrees here is an error due to the use of decrees in the preceding verse:
There is also another instance of the plural later in the larger passage:
Elsewhere the text generally uses the plural when referring to the decrees of God, but there is one instance of the singular (marked below with an asterisk):
Ultimately, the question here in Ether 2:10 is whether the original text permits number disagreement in expressions like “it is the everlasting decree(s) of God”. Given the general tendency that allows for number disagreement in the original text, it is not surprising that there is at least one other clear example in the earliest text of the phraseology “it is”:
This example shows that the plural decrees is possible in Ether 2:10 since the phraseology in Alma 30:16 (“it is the effects of a frenzied mind”) directly parallels the noun phrase form here in Ether 2:10 (“it is the everlasting decrees of God”). Also note that in the larger passage here in Ether 2, the use of the plural decrees at the beginning of verse 9 (the second section in the following citation) seems to require the plural at the end of verse 10 (the third section) as a kind of recapitulation:
Semantically, there is only one decree in this long passage, namely, “serve God or be swept off” (explicitly occurring three times, as listed above, and implied a fourth time at the end). The text nonetheless refers to this decree in the plural, not necessarily because there is more than one decree but possibly because it has been decreed more than once (to the Jaredites, to the Nephites, and now to the Gentiles). The critical text will restore the plural decrees in “for it is the everlasting decrees of God” in verse 10 since it appears to be intended.
Summary: Restore in Ether 2:10 the plural decrees, the reading of the earliest textual source (namely, 𝓟); the larger passage uses the plural decrees to refer to the Lord’s repeated decree that the people in the promised land must serve him or be swept off.