The original manuscript is not extant here in Alma 27:20, but spacing between extant fragments does not allow for the that except by supralinear insertion:
( )
THAT
-hold their joy was not that to excede their strength & now it came (t )
O PASS ^ Alma conducted his Brethren back to the land of Zarahemla even to ( ) ow(n )
HIS HOUSE &
The printer’s manuscript has the that, as do all published editions. Of course, we expect that after “it came to pass”. Yet elsewhere in the earliest text there are two examples where “it came to pass” is directly followed by the subject of the main clause without any intervening that:
In both these cases, the original manuscript is extant and the that is not there. It is quite possible that Oliver Cowdery, the scribe for these two cases, accidentally skipped the that in 𝓞 when he took down Joseph Smith’s dictation. But in copying these passages into the printer’s manuscript, it should be noted, Oliver did not insert the that. So perhaps we can assume here in Alma 27:20 that the original manuscript had a supralinear that since otherwise there is no evidence that Oliver ever added the that after it came to pass as he copied from 𝓞 into 𝓟. Of course, in the overwhelming majority of cases, we have the conjunction that between “it came to pass” and the directly following subject of the main clause. And there is evidence that Oliver sometimes initially omitted the that as he took down Joseph’s dictation:
For a fourth possibility, see under Alma 57:7; in that case, spacing between extant fragments argues for a supralinearly inserted that (just like here in Alma 27:20).
There is also one case in the history of the printed editions where the that after “it came to pass” was accidentally omitted in the context of an immediately following subject of the main clause:
In this case, the 1841 British edition accidentally omitted the that; it will be restored in the critical text.
The possibility still remains, of course, that Oliver accidentally inserted the expected that in Alma 27:20 as he copied from 𝓞 into 𝓟. Note, by the way, that in Alma 55:14, listed earlier in this discussion, the 1858 Wright edition added the that and it has continued in the RLDS text. So the tendency to add the that after “it came to pass” does exist, but the stronger tendency is to omit the that. For a complete discussion, see under that in volume 3.
Summary: Maintain in Alma 27:20 the earliest (and current) reading with the expected that after “it came to pass” and before the main clause, “Alma conducted his brethren back to the land of Zarahemla”.