Here scribe 2 of 𝓟 wrote two immediately conjoined present participial clauses but without any conjoining and: “being as numerous almost as it were as the sands of the sea / coming upon them to destroy them”. Oliver Cowdery, when he proofed 𝓟 against 𝓞, changed the second present participle coming to the finite past-tense verb form came since the subject “the Lamanites and the Amlicites” needed a finite verb form. We cannot be sure here whether Oliver’s change to came was a restoration of the reading in 𝓞 or an independent attempt on his part to remove a sentence fragment. There is evidence that scribe 2 of 𝓟 could write a present participle instead of the correct finite verb form:
In this example, scribe 2 of 𝓟 caught his error and corrected bringing to bringeth; the 1830 edition, which was set from 𝓞 rather than 𝓟 for that part of the text, also reads bringeth. We should also note that in this example from 3 Nephi 22 there was no nearby present participle that might have triggered scribe 2’s initial writing of bringing. In Alma 2:27, there is the preceding being and the following being strengthened and having prayed that could have readily influenced scribe 2 to write coming instead of the correct came.
There is one other example in the text where scribe 2 of 𝓟 incorrectly wrote a present participle:
Although in Mosiah 29:42 there is a preceding being (just as in Alma 2:27), the more probable cause for the error conferring (spelled as confering) was the immediately preceding having. Occasionally, Oliver Cowdery and Hyrum Smith also incorrectly wrote the present participle instead of the correct finite verb form; for examples, see the discussion under Mosiah 12:2.
One could argue that in Alma 2:27 the present participle coming is actually correct despite its ungrammaticality. There are, for instance, a number of examples in the original text where a present participial clause is conjoined by means of the conjunction and to a preceding finite clause (for four examples, see under Mosiah 23:13–14). But here in Alma 2:27, there is no preceding finite verb form, nor is there a conjunction and between the two present participial clauses. Thus we end up with a highly unusual construction that cannot be found elsewhere in the text. For this reason, the critical text will accept Oliver Cowdery’s correction of coming to came in Alma 2:27, under the assumption that he was probably correcting 𝓟 to agree with the reading in 𝓞.
Summary: Accept in Alma 2:27 Oliver Cowdery’s correction in 𝓟 of the present participle coming to the simple past-tense came; in this case Oliver was probably correcting to 𝓞 rather than grammatically emending the text.