Designing DynamoDB Table w/ AppSync

Question

I'm planning to implement a 1-on-1 chat app using DynamoDB and AppSync. There are a few questions in mind tho whether I'm doing the right thing or not.

Table Design

1. Is my design considered a single-table or a multi-table design?
2. The linking part for the two tables is thru the participant rows from the Chat Table. Is this correct or should i add a new field for chatId on the Users Table?
3. I'm generally new to GraphQL/AppSync. For connecting resolvers in the schema should i attach it in each fields in the query/mutations or should I attach a resolver for each field on all types and not just queries/mutations?