Here in the printer’s manuscript, scribe 2 wrote “until they knew them in full”, yet the rest of the larger passage is in the present tense, in particular in the parallel construction in the following verse:
Thus the 1830 typesetter replaced knew with the present-tense know in the preceding verse.
There are other examples of difficulties between know and knew in the transmission of the text, including this instance in 𝓟 where Oliver Cowdery himself miswrote know as knew but caught his error and corrected it:
For other examples of textually significant mix-ups between know and knew, see under Alma 38:4 and Helaman 9:36. The critical text will accept the 1830 emendation of knew to know in Alma 12:10; the original text here most probably read “until they know them in full”.
Summary: Accept in Alma 12:10 the 1830 emendation of knew to know (“until they know them in full”); the context argues for the present-tense form of the verb.