Here there are two conjoined subclauses within the larger lest-clause. The editing for the 1920 LDS edition makes both subclausal verbs take the subjunctive infinitival form (“be tempted” and “be led away”). This emended text, although fully grammatical, implies that each of the conjoined subclauses occurs independently as a consequence of not always watching and praying. On the other hand, one can readily interpret the original text as consequential, namely, with the meaning ‘lest ye be tempted by the devil and as a consequence ye are led away captive by him’. More generally, however, there are instances in the original text of conjoined subjunctive and indicative subclauses that show no particular logical connection between the subclauses. Here is an example of such a mixture of verb forms that has never been edited:
For another good example, one that has been edited, see under Alma 22:16. The critical text will restore the original are here in 3 Nephi 18:15, no matter how we interpret the relationship between the two conjoined subclauses within the larger lest-clause.
Summary: Restore the original indicative are in the conjoined subclause in 3 Nephi 18:15 (“and ye are led away captive by him”), the reading of the earliest text; usage elsewhere in the original text supports mixtures of the subjunctive and indicative within a larger subordinate clause.