For ASP.NET
Ans1:
Use RegularExpressionValidator instead of CustomValidator
Ans2:
Use RequiredFieldValidator with all the controls. And in the last use single ValidationSummary Control to display message. Here is link for you Understanding ASP.NET Validation Techniques
Ans3:
Use RadioButtonList control for options. On SelectedIndexChange event of DropdownList control populate the RadioButtonList with relevant options.
Ans4:
Link in Ans2 will give you the RangeValidator control example.