REST, gRPC or GraphQL for a New Platform

An e-commerce company is splitting a monolith into services: Catalog, Inventory, Pricing, Cart, Checkout. Consumers are (a) a public partner API used by hundreds of third-party sellers, (b) an iOS/Android app whose product page composes data from Catalog + Inventory + Pricing + Reviews, and (c) the services calling each other at ~50k requests/s aggregate.

1. Recommend a protocol for each of (a), (b), (c) and justify with concrete properties (payload, contract, tooling, caching, browser support).
2. If you pick GraphQL for (b), describe the N+1 problem that a naive products { reviews { author { name } } } resolver causes and how DataLoader fixes it. Include rough query counts for 20 products with 10 reviews each.
3. Name two other risks specific to GraphQL and one mitigation each.