For issue 1 - there seem to be a bug in the form handler on the server side, there is a field controlling  the multi-step forms that get set to -1 instead of 0 after posting the form once, we solved it like so:

    /// <summary>
    /// This circumvent a bug where the second time a form is posted, the __FormCurrentStepIndex value is -1, instead of the default 0, which causes an error in the data submit function.
    /// </summary>
    public class  CustomDataSubmissionService : EPiServer.Forms.Core.Internal.DataSubmissionService
    {
        public override SubmitActionResult PerformDataSubmit(NameValueCollection rawSubmittedData, HttpContextBase httpContext, ControllerBase controller)
        {   
            // the incoming collection is readonly, copy it
            var col = new NameValueCollection(rawSubmittedData);
            
            // the current form step should never be -1, so set it to 0 if it is submitted as such
            if(col["__FormCurrentStepIndex"] == "-1") {
                col["__FormCurrentStepIndex"] = "0";
            }

            return base.PerformDataSubmit(col, httpContext, controller);
        }
    }

And then use the custom service in your service container setup:

context.Services.RemoveAll<DataSubmissionService>();
context.Services.AddSingleton<DataSubmissionService, CustomDataSubmissionService>();

Thanks for this solution!