It is worth noting that the ID of a form can be found using the 'inspect element' tool when viewing your webpage and that any element of a form, including the "Submit" <input> tag, can be added as HTML instead of using Liquid tags by copying the HTML from the "HTML Source" of the live page and pasting it over the liquid tag.
For assistance with adding client-side validation, NationBuilder maintains a listing of certified Architects, or web developers, who are pros at making customizations to NationBuilder sites and themes: nationbuilder.com/architects
As far as server-side validation goes, this would need to be a product change made on our end, as it is not possible to add server-side validation from a NationBuilder control panel or theme. If you'd like to see additional server-side validation added to the platform, feel free to post a feature suggestion at nationbuilder.com/suggest.
Complete and current information on how to use NationBuilder is available in the documentation section.