Here in the printer’s manuscript, Oliver Cowdery wrote the word are but then crossed the e, ending up with art. When we compare this art with nearby instances of art (see, for instance, the one in Mosiah 2:21, discussed above), we find that for a true t there is virtually no loop. But for this case of art in Mosiah 2:24, the ascender for what appears to be a t is actually a rather large e that Oliver accidentally crossed.
As noted in the discussion of Mosiah 2:21, the biblically styled art occurs as a conjoined predicate but only when there is a preceding predicate that contains another biblically styled form (such as hath). Here in Mosiah 2:24, the initial predicate begins with the normal verb form are. There is no independent evidence anywhere in the text for the conjoining of predicates where one of the verbs is are and the other is art. Thus the 1830 compositor was probably correct to interpret this art in Mosiah 2:24 as a mistake for are.
There is independent evidence in the manuscripts that Oliver Cowdery sometimes wrote art in place of are, as in the following example in 𝓞 where Oliver initially wrote art, then later with somewhat heavier ink crossed out the art and supralinearly inserted the correct are:
The correction to are was probably made when Oliver read back his copy to Joseph Smith.
It appears that in two other places Oliver intended to write are but ended up crossing the e to create art, just as in Mosiah 2:24. In these two instances, the error was transmitted into the 1830 edition (and subsequently removed from the text in the editing for the 1837 edition):
See under each of these passages for further discussion.
Summary: Maintain the conjoined are in Mosiah 2:24 (“and ye are still indebted unto him and are and will be forever and ever”); the art that is found in the printer’s manuscript appears to be an are for which the e was accidentally crossed.