There's a similar answer here. It could be the emails returning an error have an issue. Try validating the email before previewing. The route you are using returns dummy data. If you want to preview for actual contact data, there are a couple of other choices.
list: 'fuelapi/guide/v1/emails/{docID}/lists/{listID}/contacts/{rowID}/preview?kind=html'
DE: '/fuelapi/guide/v1/emails/{docID}/dataExtension/id:{listID}/contacts/{rowID}/preview?kind=html'
I don't believe there is a route that allows for multiple DE's. However, you can try creating a model in Contact Builder that consolidates several DE's.
You can retrieve Journey History by POSTing to this endpoint /interaction/v1/interactions/traceevents/search
It is the same endpoint used in the History tab in Journey Builder.
Example of the request payload without any filter (apart from time)
{"from":0,"size":100,"filter":{"fquery":{"query":{"query_string":{"query":"(transactionTime:[2017-08-29T13:20:45.101Z TO *])"}}}}}
Example of the request paypload with a filter on a journey
{"from":0,"size":100,"filter":{"fquery":{"query":{"query_string":{"query":"(definitionId:74d00b19-f28f-4533-abb2-xxxxxxxxxxxx) AND (transactionTime:[2017-08-29T13:20:45.101Z TO *])"}}}}}
I suggest you use google developer tools (in Network tab) or any other monitoring tool to monitor the network traffic within the History tab of Journey Builder and learn more about this endpoint especially how to perform complex queries.
Please note that this is not officially available for use as it is not documented in the official documentation
[EDIT - 11/10/2017]
You can now also use Journey Builder Data Views to retrieve Journey activity.
[EDIT - 31/12/2021]
You can use POST /interaction/v1/interactions/journeyhistory/search or
POST /interaction/v1/interactions/journeyhistory/filter or POST /interaction/v1/interactions/journeyhistory/download endpoints.
For more information you can find all available endpoints by looking into the discovery endpoint GET /interaction/v1/rest
Best Answer
This might be of help - https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/datafolder.htm - I think it would be helpful if SFMC documentation would do some cross-linking on this topic.