Apart from the Form Events that could be used:

Form Events (IFormEntryCreatedEvent): Sitefinity Documentation, Forms events https://www.progress.com/documentation/sitefinity-cms/for-developers-forms-events

AJAX Form Events (IFormEvent): Sitefinity Documentation, AJAX forms events https://www.progress.com/documentation/sitefinity-cms/for-developers-ajax-forms-events

Additional logic can be executed in the FormModelCustom.

The form data can be accessed on submission in the FormCollection (Dictionary).

Whether the form data is persisted depends on the (IsValid) value returned by the IsValidForm method. In order to add custom validations and cancel the save operation, a custom FormModel needs to be created, inheriting from the FormModel, then the IsValidForm method overridden.

Example:
 

namespace SitefinityWebApp
{
    public class FormModelCustom : FormModel
    {
        protected override bool IsValidForm(FormDescription form, FormCollection collection, HttpFileCollectionBase files, Dictionary<string, IFormFieldController<IFormFieldModel>> formFields, List<IFormElementController<IFormElementModel>> formElements, out Dictionary<IFormFieldController<IFormFieldModel>, bool> fieldControls)
        {
            var isValid = base.IsValidForm(form, collection, files, formFields, formElements, out fieldControls);

            //Field name is MyCustomField
            //Advanced -> Model -> MetaField -> FieldName
            if (collection["MyCustomField"] == "Not a valid value") 
            {
                //Sets the validation message
                this.InvalidInputMessage = "Custom validation message";
                //controlls whether to persist the form data in DB
                isValid = false;
            }

            return isValid;
        }
    }
}

using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Frontend;
using Telerik.Sitefinity.Frontend.Forms.Mvc.Models;
 
namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
 
        protected void Application_Start(object sender, EventArgs e)
        {
            Bootstrapper.Initialized += Bootstrapper_Initialized;
        }
 
        protected void Bootstrapper_Initialized(object sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
        {
            if (e.CommandName == "Bootstrapped")
            {
                FrontendModule.Current.DependencyResolver.Rebind<IFormModel>().To<FormModelCustom>(); 
            }
        }
    }
}

public static IPAddress GetIpAddress(this HttpRequestBase request)
{
    IPAddress address = null;

    if (IPAddress.TryParse(request.Headers["CLIENT-IP"], out address))
    {
        return address;
    }
    else if (request.Headers["X-FORWARDED-FOR"] != null)
    {
        // If the X-FORWARDED-FOR header contains one or more ips - get the first one
        var ips = request.Headers["X-FORWARDED-FOR"].Split(',');
        var ip = ips[0].Trim();
        if (IPAddress.TryParse(ip, out address))
            return address;
    }
    
    if (IPAddress.TryParse(request.Headers["X-FORWARDED"], out address))
    {
        return address;
    }
    else if (IPAddress.TryParse(request.Headers["X-CLUSTER-CLIENT-IP"], out address))
    {
        return address;
    }
    else if (IPAddress.TryParse(request.Headers["FORWARDED-FOR"], out address))
    {
        return address;
    }
    else if (IPAddress.TryParse(request.Headers["FORWARDED"], out address))
    {
        return address;
    }
    else if (IPAddress.TryParse(request.UserHostAddress, out address))
    {
        return address;
    }

    return null;
}

var realIPAddress = SystemManager.CurrentHttpContext.Request.GetIpAddress();