Is "inner witness/conviction" sufficient as a criterion for inspiration?

No. Subjective feelings are changeable and culturally dependent. Inspiration is an objective gift from God, which is recognized and attested by the Church appointed by Christ.

Reformation criteria such as “inner testimony of the Spirit” or “what comforts is inspired” fall apart in practice: parts of Scripture are difficult (Books of Chronicles), while non-canonical works (e.g., The Imitation of Christ) are highly “edifying.” This shows: being moved does not reliably distinguish between the divine and the non-divine.

The Catholic route respects the inner working of the Spirit (the sensus fidei), but leaves the decisive judgment to the Magisterium established by Christ. “Faith comes from hearing” (Rom 10:17 NAB) – that is, from the proclaiming Church, not from private feeling.

Sources:

Scripture (NAB): Rom 10:17; 2 Pet 1:20–21.

Fathers/Teachers: Augustine; Thomas Aquinas (instrumental cause: God as author, man as genuine instrument).

Magisterium: Dei Verbum 10–11.