Several points to contribute to this discussion.
1) Nobody raised the issue that Elder Oaks did back in 1985 when he said we do not covenant to take upon us the name of Christ at baptism but to be “willing” to do so. If you read Section 20:37 and the baptismal prayers carefully one could conclude that we do not covenant to keep the commandments either, but only a willingness to do so.
2) Elder Oaks suggested we take upon ourselves the name of Christ in the temple. I would add we commit to obedience too. Maybe there is a grace period for newly baptized people to grow toward taking upon themselves the name of Christ and promising to keep the commandments, and that becomes a reality in the temple.
3) President Lee actually said that the temple covenants "which ... are ... an embodiment or and unfolding of the covenants each person should have assumed at baptism." Clyde J. Williams, ed., The Teachings Of Harold B. Lee, (Salt Lake City: Bookcraft, 1996), p. 574.
4) There are numerous statements now that we renew all covenants we have made when we partake of the Sacrament. The earliest I know of is 1965 by Elder Stapley and the latest is 2015 by Elder Ballard. A number in between.
5) This all suggests that the Church (and its leaders) may be growing in their understanding of the details and technicalities regarding the covenants of baptism and sacrament (once upon a time didn’t they do rebaptisms to renew the covenant, which also suggests they didn’t understand things the way we do now?), and so it seems possible to me that it is perfectly legitimate for modern living prophets help us understand that Mosiah 18 is a part of the baptismal covenant as part of this growth in learning that is taking place in the Church.
I would add in this regard that the most recent book by Bruce and Marie Hafen, Covenant Hearts, goes a long way to suggest there is a considerable difference between the laws and ordinances which are associated with the Aaronic and Melchizedek Priesthoods that seem to be relevant to these issues as well.
