𝓞 is not extant here but probably read “even unto boasting in my God”; there is no room between extant fragments of 𝓞 for the definite article the before boasting except by supralinear insertion. In the 1837 edition, this standard gerundive form was changed to the mixed gerundive form “even unto the boasting in my God”. The original text had quite a few examples of this mixed gerundive form, as explained under 1 Nephi 17:32. For another example where the 1837 edition added the to create an example of the mixed form, see under Alma 12:22. For a general discussion of this kind of nominal construction, see under gerundives in volume 3.
The 1908 RLDS edition removed the intrusive the here in Alma 26:35, probably by reference to 𝓟; the 1920 LDS edition made the same change but apparently by reference to the 1830 edition. The critical text will also follow the earliest textual sources by avoiding in this instance the intrusive the.
Summary: Maintain in Alma 26:35 the earliest attested gerundive form boasting without the preceding definite article the (thus “even unto boasting in my God”).