Here the earliest text reads anomalously as “and I also rememberest”. The -est ending could be an error caused by the use of the second person singular ending -(e)st in the surrounding text, “thou didst shew thyself unto them in great power … that thou hast said that thou hast prepared a house for man”. We can find evidence elsewhere in the text for similar errors based on the inflectional ending -(e)st in the surrounding text:
In these two cases, Oliver Cowdery wrote sayest in 𝓞 instead of the correct saith. In the first case, he caught his error in 𝓞; in the second case, he made the correction when he copied the text from 𝓞 into 𝓟. Similarly, here in Ether 12:32 Oliver, the scribe in 𝓞, could have written rememberest in 𝓞 instead of remember. Of course, it is also possible (since 𝓞 is not extant for Ether 12:32) that Oliver could have made the error when he copied the text from 𝓞 into 𝓟. Interestingly, the 1830 compositor set the difficult rememberest. In his editing for the 1837 edition, Joseph Smith made the change to the standard remember.
The two changes in Alma suggest another possible emendation, namely, 𝓞 could have read “and I also remembereth” rather than the standard “and I also remember”. There is, for instance, evidence that the original text permitted the expression “I saith” for the historical present (see the discussion under 1 Nephi 11:3). In addition, there is evidence that the original text allowed the occasional use of the ending -eth for the first person singular I:
Here in Ether 12:32, the intervening also comes between the subject I and the verb, thus allowing for the verb form remembereth, just as in both of the above examples there are some intervening words between the initial I and its associated verb.
It seems fairly obvious that rememberest is an error in Ether 12:32, but it is not clear whether the original verb form should be remember or remembereth. It is even possible that the original text itself read remember but that 𝓞 incorrectly read remembereth. Elsewhere, however, the text consistently prefers “I remember” over “I remembereth” (5 to 0), including one case with an intervening also (marked below with an asterisk):
Note especially the nearby example of remember in Ether 12:33 (“and again I remember that thou hast said that thou hast loved the world”); in that case we again have two instances of hast in the following subordinate clause, yet in that case remember was not changed to rememberest. This example in verse 33 argues for remember rather than remembereth as the original reading here in verse 32. The most plausible solution in Ether 12:32 is to assume that the original text read “and I also remember” rather than “and I also remembereth”. The third possibility, “and I also rememberest”, the actual reading of the earliest text, is most probably an error caused by the surrounding instances of didst, hast, and hast.
Summary: Accept Joseph Smith’s emendation of rememberest to remember in Ether 12:32; the -est ending appears to be an error due to the instance of didst earlier in the passage and to two instances of hast in the immediately following subordinate clause; usage elsewhere in the text argues against remembereth as the original reading in this passage.