The earliest text here has the subject form who even though the object form whom is standard (note in verse 18 the object form me in the clause “ye have called me your king”). Here in verse 19, the standard whom was introduced into the text in the 1837 edition.
The original use of the subject form who may have been influenced by the who that occurs in the immediately following relative clause (“who has spent his days in your service”). Nearby in king Benjamin’s discourse, we have only the standard “I whom” for the very same relative clause:
These two examples suggest that the who in verse 19 is an error for whom.
Even so, the original text does have other cases where the earliest text has a nonstandard use of who in place of the standard whom:
These examples from the earliest text suggest that the occasional nonstandard use of who should be accepted in the critical text, even though such instances could be due to grammatical error on the part of either Joseph Smith or his scribes. For further discussion, see under pronouns in volume 3.
Summary: Accept the original nonstandard use of who in Mosiah 2:19 (“if I who ye call your king”).