Maybe I'm missing something...but is the email app read only? I swipe and get the options to flag, mark unread, or delete. I force touch and get the same options: flag, marke unread, archive.
The Apple mailbox won't allow replies, but there are third party apps that will give you this capability. I'm currently trying out CloudMagic which gives the option to reply either with a canned response or to dictate one. It definitely works, but apart from a few test mails I've not actually felt the need to use the reply feature yet. However, I can see that in some situations, it could be really handy.
The current workflow is have Mail open on your Watch. Then, from your phone with Handoff, the Mail icon will show on the lock screen. Swipe up and unlock, and you'll be in your email at that message, ready to reply.
Maybe I'm missing something...but is the email app read only? I swipe and get the options to flag, mark unread, or delete. I force touch and get the same options: flag, marke unread, archive.