@aferlazzo Well, first you need to make sure you are not changing or refreshing the page when you submit the form: https://stackoverflow.com/questions/19454310/stop-form-refreshing-page-on-submit
If you do that then you can easily show a dialog or something if an input is invalid. The page transitions are controlled with ons-navigator components.
For the unformatted text, perhaps try using local dependencies so the CSS is loaded earlier. Otherwise, you will probably have a loading screen at the beginning of your app that will hide all the “set up”.