Ross Geddes (personal communication, 28 November 2004) suggests the possibility that the plural thoughts in this passage could be an error for thought. If the earliest text were emended in that way (to “this is my thought”), then the disagreement in number would be eliminated with the least change. On the other hand, Joseph Smith, in his editing for the 1837 edition, eliminated the disagreement by changing the subject and the verb to the plural (to “these are my thoughts”).
Under Mosiah 28:3, I discuss a number of cases where the Book of Mormon text uses the plural thoughts instead of the singular thought, including cases like “the very thoughts” (where modern readers expect the singular). The plural thoughts is probably correct here in Ether 2:15 and will therefore be retained in the critical text. A syntactically similar example is found in the preceding book:
As explained under that passage, the original text very likely read “this is the words”. In addition, as discussed nearby under Ether 2:10, there are other examples in the original text of disagreement between the subject and its complement, as in Alma 30:16 (“it is the effects of a frenzied mind”) and in Ether 2:10 (“it is the everlasting decrees of God”).
Summary: Restore the singular subject this and the singular verb form is in Ether 2:15 while maintaining the plural thoughts, thus giving one more case of disagreement between subject and complement in the original text (“this is my thoughts upon the land which I shall give you for your inheritance”).