How exactly does copying a spell into a ritual spell book work with regards to the ritual casting feat? Does it use the spell scroll in the process? I know copying a scroll destroys it if you copy it into a wizard spell book — does that apply here?
Yes, spell scrolls are always destroyed by copying. From the DMG entry on spell scrolls, page 200:
When a spell is copied from a spell scroll, the copier must succeed on an Intelligence (Arcana) check with a DC equal to 10 + the spell’s level. If the check succeeds, the spell is successfully copied. Whether the check succeeds or fails, the spell scroll is destroyed.