The and that was originally at the beginning of the after-clause was removed by Joseph Smith in his editing for the 1837 edition. But no matter what one does, Mormon’s text here ends up creating an incomplete thought. At the beginning of this verse, in reference to king Mosiah, Mormon writes “therefore he took the records”; but then Mormon gets diverted from telling us what Mosiah did with the records and starts discussing the translation of the Jaredite record. Finally, in verse 20 Mormon returns to his original idea that he introduced at the beginning of verse 11 and completes what he originally wanted to say about Mosiah transferring all the records to Alma:
The removal of the and from before the after-clause in verse 11 doesn’t really help anyway since verses 11–19 lead to a diversion from what Mormon originally intended to say. One advantage of keeping the and is that it forces the reader to keep waiting for some semantic closure (which finally comes in verse 20). In the original Book of Mormon chapter system, verse 20 occurred at the beginning of a chapter (namely, Mosiah XIII). When Orson Pratt versified the LDS text for the 1879 edition and redid the chapter system, he put the first part of the original chapter XIII at the end of his new chapter 28. Apparently, his idea was to make sure that the topic introduced in verse 11 of his chapter 28 would be semantically completed within the same chapter.
Summary: Restore the and in Mosiah 28:11; this additional and helps to maintain the incomplete nature of the textual diversion in verses 11–19; only in verse 20 is the topic that Mormon introduced in verse 11 finally completed.