I do not have idea about how to display InfoPath Form in the email body but you can surely embed the URL of Active InfoPath Form. Select "Set Variable Workflow" action in SPD and set the variable to the string specified in the above image.
First select [%Workflow Context:Current Site URL%] by clicking "Add or Change Lookup" button and select Data Source: Workflow Context; Field from source: Current Site URL, then same string as it is till Cases/, then select Current Item:Title, then write "&Source", then again [%Workflow Context:Current Site URL%] and finally write or copy the string "/Cases/Forms/AllItems.aspx&DefaultItemOpen=1"
In the second image, you can see there a link "here", by clicking this click an active InfoPath form will be opened in the web browser. [Just confirm that your InfoPath form is a web browser enabled].
You can include this "here" link by clicking the small icon next to "Automatic"