Salesforce Sales Cloud - How can I ensure that the oauth flow redirects to my sandbox for Salesforce?

You can add a siteAddress parameter to the GET /sfdc/oauth/url. For example:

https://api.cloud-elements.com/elements/api-v2/elements/sfdc/oauth/url?apiKey=<key>&apiSecret=<secret>&callbackUrl=https://console.cloud-elements.com/elements/jsp/home.jsp&siteAddress=https://test.salesforce.com>