I am not clear why you link any problem you have with your Nabble app to it being embedded in a Weebly site. It appears to be displaying exactly as expected.
The "cookie screen" text wraps perfectly.
I agree rhat there is a problem on the main screen as the
Subscribe via email does not wrap, but this appears to be a non-standard feature that you have added and it will be down to you to code it so it wraps. In any case this is not related to Weebly or even embedding in general.
On the more general issue of the use of Nabble on narrow screens, then I wonder why you have not made use of the standard feature at:
Options > Application > Change appearance > Preferences > Page layout > Narrow
This will remove the sidebar and place those links in a horizontal format.
Beyond that we need to see some dummy posts which show features you complain about.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.