First question: I can no longer change regular expression through any behavior. Is this correct? DataAssist builds this at the same time. But I must be able to change this afterwards, right?
Yes. You can edit the Server Validation server behavior to add Regular Expression validation after running the wizard. Regular Expression validation isn't exposed in the wizard itself.
Second: Can I change the look of JQuery Validation afterwards? Or remove it completely, to use Bootstrap's validation instead?