We don't know for sure if Shana is Mathilde's daughter, that's only my impression because:
1. Shana was born around the time Mathilde died.
2. Shana conveniently had all the powers necessary to become the ideal host for Alastor.
3. Mathilde loved Alastor, Alastor probably loved her back.
Yuuji's transition from Mystes to Tomogara happened to moment Hebi merged with him, but I'm saying if you take away the Hebi, he's just a Tomogara with Kami level strength, three Hougu and some fighting skills/a few elementary spells.
With the Hebi attatched, he has twice his original strength (2x infinity), a whole library of spells, all of Ball masque at his command, the Haridan and his own personal Stand, the semi-sentient Knight of the Silver Flame.