@marcin.nowak I cannot fully answer your first question, but the same problem happens when rendering dialogs: https://community.developer.atlassian.com/t/extra-padding-on-adg3-dialog/4896/4
I hope this can be improved soon.
Regarding your second question: I don’t think Atlassian intends to provide anything like that. All previous discussions I have seen in this forum stated that there is no plan to add such a feature.
See: https://community.developer.atlassian.com/t/update-on-how-to-detect-adg3-enabled-tenants/1275