you can use mustache in the email body to access arbitrary data from the input. See the example in the docs. In order to access the first searchhit, you could try ctx.payload.hits.hits.0._source.@timestamp.
can you retrieve the source of that document by calling GET index/doc/ID_OF_DOC/_source and GET index/doc/ID_OF_DOC - looks as if there is no JSON in that document.
Also please include the whole query here or better yet the whole watch and the whole execute watch API output instead of snippets. This would ease debugging tremendously.
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