The typesetter for the 1849 LDS edition accidentally replaced those with these. The subsequent LDS edition (1852) followed this reading, but the 1879 LDS edition restored the original those. In this context, those is the better reading; the determiner these implies that those who died “with old age” also died “in the faith of Christ”, which was not necessarily the case. There are quite a few examples in the history of the text of mix-ups between these and those; for a list of examples, see under Mosiah 28:1.
Summary: Maintain the original reading with those in Alma 46:41: “and those who died in the faith of Christ are happy in him”.