There are two different scribal errors that Hyrum Smith made in this verse. First, he twice wrote the standard adverbial always (which ends in s) as the s-less adverbial alway (he spelled it as allway). In the second instance of allway, he virtually immediately corrected his allway by adding the s, but in the first instance he left the s-less form in the manuscript. Of course, the 1830 compositor set always. Earlier in verse 12, Hyrum twice wrote the adverb as allways and without correction. Oliver Cowdery once wrote always initially as alway but quickly corrected it to always (in 3 Nephi 18:7). Otherwise the scribes consistently wrote this adverb with the s.
The Oxford English Dictionary indicates that the s-less form alway is basically an archaic form that survives in poetry, as in this example from Richard Chenevix Trench’s Poems from Eastern Sources (1842): “and boldly use the children’s prayer alway”. The s-less form alway is found in the King James Bible, occurring 23 times (versus 62 times for always). The s-less form that Hyrum wrote could possibly reflect some dialectal influence or even his familiarity with the King James Bible. More likely, he accidentally omitted the s at the end of always as he copied the text from 𝓞 into 𝓟.
The second scribal error that Hyrum Smith made in this passage occurred when he initially wrote the first part of the verse without the word always (“if this could be [the case]”). It’s also possible that he started to write “if this could be [always the case]” (that is, with a different word order). In any event, he crossed out the initial be and wrote always be inline (but with allway instead of the correct always). The original syntax undoubtedly read as corrected in 𝓟.
Summary: Maintain in Mosiah 29:13 the immediately corrected syntax in 𝓟: “if this could always be the case”; also maintain the form always, not the possibly archaic or dialectal alway that was accidentally produced twice here by Hyrum Smith in 𝓟 (spelled as allway).