You will find the documentation on the webmail features at this link:
https://help.runbox.com/documentation/
You should find most of what you are looking for there, including the reference to clicking on the sender as a short cut to starting a reply which is documented here:
https://help.runbox.com/runbox-webmail/
There isn't an option to not include the previous message in a reply, but I will pass this on as a suggestion to the development team.
Please just ask any questions you may have as they arise also.