The option I can offer up is potentially modifying the code to display a larger button.   Outside of that, I'd need to lean on the larger community here - this isn't something I see often.  

In the IDE we would need to potentially modify the default HTML Template or one of the files.  Ill see if can find out more.  

If a single response is all that is needed, and any of the recipient can click it (i.e. no authentication) Pending Actions will work

Playbook loops could handle this in Series. However each send/receive is in order, so user1 must complete before user 2 is informed

Given URL links in emails are restrictive, it's impractical to smuggle logic/JS into that to hit the SOAR API direct

Beyond this I would say either a link to a specific Google Forms (or other) for the user to acknowledge. This way the Google Form could in theory be generated dynamically, and the appropiate link is sent.  Then either you collect the Google Forms responses and find a way to merge the responses into the Case grouping which would require some playbook logic, or have the playbook scrape an API to get the status of all forms responses. However I don't know how much of that workflow is OOTB

I looked at a couple of other routes but ruled them out. 

HTH Andy