In this passage, the original text has two instances of saith (spelled as sayeth in the manuscripts). The first, “and Ammon saith”, is an instance of the historical present tense; this saith was edited to the past-tense said in the 1837 edition, in accord with Joseph Smith’s general editing for that edition (see the discussion under 1 Nephi 2:1 as well as more extensively under historical present in volume 3).
In addition, Joseph Smith edited the second saith from the indicative to the subjunctive (giving “and if he say unto us” in the 1837 edition). This subjunctive reading has been retained in all subsequent editions except for the 1953 RLDS edition. That edition accidentally changed the subjunctive say to the past-tense indicative said, undoubtedly under the influence of the preceding occurrence of the edited said in this passage (“and Ammon said”).
Joseph Smith marked the change of the second saith to say in 𝓟, so that change was clearly intended. And in one other long passage, Joseph repeatedly edited the indicative “if he saith” to the subjunctive “if he say”:
In fact, in the second instance (in Helaman 12:14), the earliest text reads in the subjunctive (“yea if he say unto the earth”). In that case, the 1830 compositor set saith, probably because in 𝓟 the three other cases read saith (although in the first instance, in Helaman 12:13, Oliver initially wrote say in 𝓟 rather than saith). For further discussion of these four instances of saith/say, see under that passage in Helaman 12.
There are, however, two verses where Joseph Smith did not grammatically emend the indicative “if X saith” to the subjunctive “if X say”; in fact, these two examples occur right after the one here in Alma 27:7. In each of the two cases, there is first an historical present-tense saith, which Joseph changed to the past-tense said (as expected). But in each case he left unchanged a following indicative “if X saith” (each case of “if X saith” is marked below with an arrow):
In general, the text has examples of both the indicative and subjunctive in if-clauses. One interesting example conjoins both subjunctive and indicative verb forms, neither of which have ever been edited:
In two other Book of Mormon examples, a King James passage (Micah 5:8) is quoted, first in the indicative, then in the subjunctive:
Micah 5:8 has the subjunctive go; it is possible that the goeth in the first Book of Mormon quotation is due to the following treadeth and teareth.
In general, the text allows for either the indicative or subjunctive in if-clauses. For each if-clause, the critical text will follow the reading of the earliest textual sources in determining whether the verb should be in the indicative or in the subjunctive. Here in Alma 27:7, the original indicative saith will be restored in the critical text. For further discussion, see under subjunctive in volume 3.
Summary: Restore in Alma 27:7 the original indicative saith in the if-clause; the original historical present-tense saith will also be restored.