Hi @marc!
Just to quickly answer the question regarding the clientKey - yes, it will stay the same. In fact, irrespective of migrations, the clientKey should be persistent through installations and uninstallations. The only scenario a clientKey can change is during a site import which is outlined on this page.
If you do find the clientKey is changing for other reasons, please do reach out so we can explore further as that is not expected behaviour.
Thanks a lot!