The original manuscript is not extant here, but there is definitely room for & ever after forever:
( f)or our joy is full yea we will praise our God for THE LORD YEA WE WILL REJOICE
( n g)lory to much in the Lord yea who can say to much
-EVER < > BEHOLD WHO CA
The question here is whether the conjunctive phrase & ever (if it was in 𝓞) was crossed out in the original manuscript. It is possible that Oliver Cowdery, when copying from 𝓞 into 𝓟, simply missed the extra & ever. Of course, he may have accidentally added it in 𝓞, at least initially, since the expression “forever and ever” seems expected.
What is surprising is that everywhere else in the Book of Mormon text, when the context refers to the praising of God, we never have “forever and ever”, only “forever”:
Also note that two of these occur nearby in verses 12 and 14 of Alma 26. The critical text will therefore retain the reading of the earliest extant source, in this case 𝓟, for verse 16: “we will praise our God forever”.
As David Calabro points out (personal communication), there are other possibilities for what Oliver Cowdery might have written between extant fragments of 𝓞. For instance, here in Alma 26:16 Oliver might have written & now behold instead of simply behold. In general, one cannot be sure what might have been written in the lacuna; normally the critical text will accept the earliest extant reading (usually the reading in 𝓟) unless there is clear evidence that that reading is in error.
Summary: Maintain in Alma 26:16 the reading “we will praise our God forever”; in accord with the consistency elsewhere in the Book of Mormon, the original text probably read without and ever after forever in this reference to praising God; if Oliver Cowdery initially wrote forever & ever in 𝓞, he probably crossed out the extra & ever.