Actually, Gilver's armor in the novel isn't described as looking like Nero Angelo's. The most "match" there was was when Nell sees his shadow and it's described as an "armored Knight". Plus he never wore the helmet, nor did Dante ever DT in the novels.
Yeah, there is some inconsistancies, due to DMC3's writers NOT DOUBLE CHECKING established continuity. However, before Kamiya had left capcom, he acknowledged the inconsistances and even said that as far as Capcom had been concerned with the DMC canon up to DMC3's point, the mangas were canon, the novel was canon, and even DMC2 counted. So yes, the character and background info as established in the prequel novels is still canon; Dante once was known as Tony Redgrave; Nell Goldstein made Ebony and Ivory for him. He loves Sundaes, Vergil once covered his identity as Gilver, they met at least once a year prior to DMC3. Yeah, his age is revised from an vaguely implied mid20s to 17, and the art does show some version of Force Edge, but since the narrative itself never describes his sword to look like Force Edge, you can just imagine it's Rebellion in closed state.