I spent several hours this morning trying various things and the only way I could get it to not render those border width settings in line was to set EnableTheming="false"
The config setting doesn't really work there, when I had EnableTheming='<%# ConfigHelper.GetBoolProperty("EnableThemeInEventCalendar", false) %>' it always seemed to be resolving to true because the border width is always there in line style but setting it to false explicity removes it.
What I finally arrived at is this:
<mp:DataCalendar id="cal1" runat="server"
Note that I did remove BorderStyle and BorderWidth but had to leave some of the other settings because otherwise there are unintended cosmetic side effects that can only be fixed by enabling theme again and this would put the inline borders back. So this is the best compromise I could come up with.