Mobile menu and navigation design for a complex site

We are looking for an experienced designer to rework the mobile menu and navigation of a large, multi-level website. The project has hundreds of sections, a filterable catalog, a member area and a help center, and the current menu simply cannot keep up: visitors fail to find what they need, get lost in nested subcategories and leave before finishing a task. We need someone to rethink the information architecture, group sections around real user scenarios, and propose a clear navigation structure that works on screens from 320 pixels wide and up. Gestures matter a great deal: a slide-in side panel, swipes, back navigation, closing by tapping outside the area, and sensible behavior while scrolling. Every state must be fully designed — open, closed, loading, error, empty section, active and visited item. We expect serious attention to accessibility: generous tap targets, sufficient text and background contrast, screen reader support, keyboard navigation, a visible focus indicator and meaningful labels. From the contractor we need interactive mockups, a clickable prototype of the key flows, and a developer specification describing animations, transitions and component behavior. Experience with comparably complex products is required; please attach links to examples and a short description of how you approach this kind of work.