Preserving the actual words of each route and reconciling an apparent discrepancy through evidence are separate tasks. Al-Nawawī supplies the genealogical facts that reconcile the two descriptions of al-Qāsim; one need not leave the apparent contradiction unresolved, but one must not manufacture a single combined wording.
There is also an isnād objection: why does Muslim cite Abū ʿAqīl, the associate of Buhayyah, when criticism of him was reported? Al-Nawawī offers two answers: (1) Muslim may not have regarded a substantiated, explained jarḥ (jarḥ mufassar) as established against him; on the principle stated here, an unexplained charge is insufficient. (2) Muslim may have cited this route for supporting corroboration (mutābaʿah/istishhād), not as the independent foundation of proof. In such a supporting role a report may be adduced from a transmitter whose narration would not independently suffice. These answer this particular objection; they are not blanket authentications of every chain.