The original manuscript definitely has the plural plans, but Oliver Cowdery copied it into 𝓟 as the singular plan, which is consistent with other usage in the Book of Mormon. God’s plan with respect to the atonement is referred to 29 other times in the text; nowhere else is it referred to as God’s plans. Here are the examples that refer to God’s plan as a great plan (like here in Alma 34:9, except for grammatical number); two of these examples are from this same chapter of Alma:
There is also one other instance of “great plan”:
But there are no other examples of “the great plans of X”.
When plan is postmodified by an of-prepositional phrase, we usually get a noun that refers to the atonement:
“plan of redemption” 17 times
“plan of mercy” 3 times
“plan of salvation” 3 times
“plan of happiness” 2 times
“plan of deliverance” 1 time
“plan of restoration” 1 time
In the earliest text, there are only three instances that refer to “the plan(s) of God”, counting the one here in Alma 34:
So when the postmodifying noun phrase refers to God directly, there are two instances with plan and one with plans (but only in 𝓞). When we compare this result with cases of “the plan(s) of X”, where X refers to an adversary, either Satan or a person, we get the same statistics, two with singular plan and one with the plural plans:
For these three instances, the opposing grammatical number is impossible because of the determiners: “that cunning plan”, “this was a plan”, and “all the secret plans”. So in theory, God can have “great plans”, although here in Alma 34:9 this may be an error for “great plan”.
Elsewhere in the text there are no passages where plan and plans have ever been mixed up in the history of the text. But in general, we know that Oliver frequently mixed up singular and plurals in 𝓞 and 𝓟. For some discussion, see under 1 Nephi 2:5 (regarding borders and shores). Thus plans in 𝓞 could be an error for plan here in Alma 34:9.
The plural reading plans does seem wrong here in Alma 34:9, given that elsewhere the text consistently uses the singular plan to refer to the plan of salvation. Nonetheless, plans is not impossible, and it is the clear reading in 𝓞. The critical text will therefore restore the earliest reading, plans, even though the chances are high that it is an error for plan. For a similar example, see the discussion under Mosiah 16:1 regarding the clause “he stretched forth his hands” (where the earliest textual source reads in the plural rather than the expected singular).
Summary: Restore in Alma 34:9 the plural plans in “according to the great plans of the eternal God” (the reading in 𝓞), despite the possibility that plans could be an error for plan.