Here in the original manuscript, Oliver Cowdery initially wrote who is: “this people who is a branch of the tree of Israel”. He probably wrote the who because of the preceding who in “my God who hath been mindful of this people”. In this case, either who or which is theoretically possible in the original Book of Mormon text since the biblical style allows the relative pronoun which to refer to persons (see the discussion under which in volume 3). For this relative clause in Alma 26:36, Oliver also initially wrote the verb as the singular is. As explained under Alma 24:30, the noun people can be treated as either a singular or a plural in the Book of Mormon text.
Here in Alma 26:36, the original reading is in the plural: “this people which are a branch of the tree of Israel”. Oliver Cowdery’s correction of his mistake in 𝓞 is an immediate one: he erased the i of the is, then overwrote the o of who and the word is with the ich of the which (the ich ended up being written with somewhat heavier ink flow, perhaps because of the erasure); Oliver also supralinearly inserted the correct are, but that correction has the same level of ink flow as the original inline text.
Interestingly, here in the original text the conjoined predicate in the second relative clause began with hath: “this people which are a branch of the tree of Israel and hath been lost from its body in a strange land”. In the original text of the Book of Mormon, the inflectional ending -(e)th (as in hath) can take either a singular or a plural subject. Here in Alma 26:36, in his editing of 𝓟 for the 1837 edition, Joseph Smith replaced the historically singular hath with the plural have, in agreement with the preceding are: “this people who are a branch of the tree of Israel” (Joseph’s edited text in 𝓟). For some reason, the 1837 edition ended up replacing have (originally hath) with has, thus conjoining a singular verb form with a plural one: “who are a branch of the tree of Israel and has been lost”. (As explained under Alma 24:30, this kind of mixture in number for the noun people does occur elsewhere in the text.) The critical text will, of course, restore the original hath here. For further discussion, see under subject-verb agreement in volume 3.
Summary: Maintain in Alma 26:36 the corrected reading in 𝓞: “this people which are a branch of the tree of Israel”; also restore the original hath in the following conjoined predicate: “and hath been lost from its body in a strange land”.