The ungrammaticality of the earliest text for this passage led the editors for the 1920 LDS edition to remove the first when. The original manuscript is not extant here, but spacing between the extant text argues that this when was in the original manuscript. Another possible emendation would be to add the pronoun they before the second came and treat the second when-clause as parenthetical:
The original manuscript is extant right before came forth, and there is definitely no subject there. The style for this second possible emendation is somewhat more awkward than the 1920 emendation.
There is strong internal evidence that the 1920 emendation is indeed the original reading here in Alma 47:28. When we consider all other when-clauses in the original text (631 of them), we find no instance where the subject is ellipted from the main clause but is found within the when-clause. Yet this is what we have in the earliest reading for Alma 47:28: “and it came to pass that when all they who loved the king when they heard these words / came forth and pursued after the servants of the king”. The earliest reading here is both unique and unusual. The 1920 emendation suggests the possibility that when was accidentally inserted after “and it came to pass that”, a common enough place for when to occur (118 times elsewhere in the original text).
It is also possible that the second when, which is clearly part of the original text, influenced Oliver Cowdery to accidentally write an extra occurrence of when in 𝓞. There is, in fact, one example in the history of the text where an extra when was inserted in the environment of the phrase “it came to pass that”, although in this case before the phrase:
This error created a nonsensical reading in the 1840 edition, yet it was retained in the RLDS textual tradition until the 1908 RLDS edition. We also have an example where Oliver Cowdery anticipated a following when as he copied from 𝓞 into 𝓟:
These two examples argue that an anticipatory when could have been inserted in Alma 47:28 during the early transmission of the text.
Summary: Accept in Alma 47:28 the emendation in the 1920 LDS edition as the original reading: “and it came to pass that all they who loved the king—when they heard these words—came forth and pursued after the servants of the king”; Oliver Cowdery appears to have accidentally inserted an extra when in 𝓞 in anticipation of a following when and in an environment where when occurs quite frequently.