You are correct regarding the mismatching of HTTP and HTTPS.  If SharePoint 2013 is using HTTPS then K2 should use it as well.

In regards to using the same web site for all K2 components it would not be the recommended approach.  Putting workspace in its own site and SmartForms (Designer and Runtime) in its own site will allow you more flexibility later down the road.  What you could do is add a host header value to each of the K2 sites for the URL you would like to use and specify HTTPS along with the certificate you would like to use.  Once that is done you can run BlackPearl Setup Manager and then SmartForms Setup Manager again in that order.  Once you get to the site details for each manager you can click on the Bindings section and check the box for Use and Default so that it uses your new host header value.

For the actual SSL certificate it is recommended to use a wildcard if you have one as it will cover all sub domains for a given domain (i.e. *.contoso.com would cover both K2 sites).  If that is not an option a SAN certificate would work as well as it can cover several domain names.  If you are looking to use one certiifcate for both SharePoint and K2, however, a wildcard is pretty much going to be your only option as that is required for the app model in SharePoint 2013 if you are using https.

Please let me know if this helps.

Thanks,

Jimmy