Spark was built to be used with a keyboard, you can hit CMD+K to go anywhere. With Spark you can fully navigate snooze and send later features without a mouse but it doesn't support natural language and this is where it got a bit clunky as if you want to snooze on a custom date, you need to count the exact days into the future rather than being able to type it in.
If you want to reply in line, the process is that you need to copy the entire email, format it, and then use their quoting feature and then write your reply. This is tedious.
Spark has templates, and you can access them via your keyboard using the shortcut Cmd + Shift + T, and it has placeholder functionality. You can also save any To, CC, and BCC fields to your template turning them into a bit of a workflow.
Sparks offline mode is good, it doesn't need to be enabled although it only caches recent emails and users complain its quite slow.