I'm assuming you're coming at this from a developer perspective and are willing to make some changes to your ascx controls.

The answer to your first question is twofold.  To make sure that your controls only submit to their intended buttons, there is an IE-specific solution, as well as a more general solution (I usually make use of both).  For IE, you can set a default button for section of your page by setting the DefaultButton property on an asp:Panel that surrounds your content.  For non-IE, you can use the DotNetNuke.UI.Utilities.ClientAPI.RegisterKeyCapture method (from the DotNetNuke.WebUtility assembly), passing in your asp:Panel from above, the button to submit to, and the key code for enter (13, or System.Windows.forms.Keys.Enter from the System.Windows.forms assembly).

In terms of validators, assign a ValidationGroup to each validator and the submitting button, so that the validation won't "cross-pollinate."

Hope that helps,