Sorry not sure what you mean.

When it comes to setting webcat as default browser please try using the command line argument --set-default with harbour-webcat.
So execute for setting webcat as default as it also creates the apprpiate dbus config files so that a new page is opened even if webcat is already running and a link is clicked.

To switch back to the default browser use
Mimer won't deal with dbus stuff but only set the app as default. Which is a little bit unhandy with the browser as a click on a link when webcat is already open would try starting another instance of webcat. This fails however as the WaylandClient / Lipstick already sees an instance running and switches to it.