Scriptural language typically refers to “in a sepulchre” (see, for instance, the examples listed above under Alma 19:1). But the earliest text in Alma 19:5 reads “in the sepulchre”. Here the 1874 RLDS edition replaced the definite article the with the indefinite a, which will work, of course. The RLDS text has retained this secondary reading. But there is nothing wrong with the definite article here in verse 5. Earlier in Alma 19:1 the text had already referred to the sepulchre (“they were about to take his body and lay it into a sepulchre”). So the use of the definite article later on is perfectly acceptable. In fact, the queen may have simply been referring to a generic place of burial in verse 5. The critical text will maintain the original the here in Alma 19:5: “and that he ought to be placed in the sepulchre”.
Summary: Maintain in Alma 19:5 the definite article the before sepulchre, the earliest reading.