Hi partner,
Thanks for your invitation,
1.About updating case description based on the latest email.
Only when an email's regarding field is related to this case, then this email will show on the timeline. So you could use workflow to update the case automatically.
a).Set the trigger condition.


the email receiver should be the customer of the case.
b).Create an update action.


map the description of email to case description.
Note that this function is only available when you sending and receiving emails in Dynamics 365. If the email is from outside, you should use Dynamics 365 app for outlook to track the case for the email so that the "regarding" feild will be related to the case, otherwise the process will throw error.
2.About set the email content to case description.
I saw a similar issue in another thread, and the customer finally resolved the issue by replacing the mapping field from "description(email)" to "EmailMessage(Email)".
https://community.dynamics.com/365/customerservice/f/dynamics-365-for-customer-service-forum/371033/strange-text-showing-in-the-front-of-all-email-case-descriptions
But I then tried in my instance with "description" and "EmailMessage", I found a strange thing that when I use "description", then it showed me orginal html code like yours. And when I use "EmailMessage", it showed the Subject of the email and without other code.
Unfortunately, due to some rules of D365, we could not get the email content directly without coding. The more common method at present is still using custom workflow with C# to do this.
https://community.dynamics.com/365/financeandoperations/b/nareshsdynamicsaxblog/posts/create-custom-workflow-in-d365-dynamics-365
https://zapobjects.com/microsoftdynamics365_crm_email_parser_app/implementing-email-case-functionality-simple-way-using-zap-email-parser-microsoft-dynamics-365-crm/
But in your case, I found another easier way to do this. We could use JS code to extract the message content from the HTML code and make it as a value to reset to the description field.
Please add this js function to the form onload event. And it means you need to open the case form at least one time to convert the original html code to correct content in decsription field.
function getContent(executionContext){
var formContext = executionContext.getFormContext();
var content=formContext.getAttribute("description").getValue();
var content=content.split('mal">')[1];
var content=content.split('<o:p>')[0];
formContext.getAttribute("description").setValue(content);
//alert(content);
}
Original:

Now:

Hope it helps,
Best Regards,
Leo