How to put an Asp.net Core Web API response in a Bootstrap message on the client side

Good day all:

I'm struggling the following

1. I have a ASP.NET Core web API that provides User registration as follows

 public async Task<IActionResult> RegisterUser([FromBody] NewUser user)
 {
           
         User appuser = new User()
         {

             UserName = user.Username,
             Email = user.EmailAddress,
             FirstName = user.FirstName,
             LastName = user.LastName,
             Address = user.Address,
             PhoneNumber = user.PhoneNumber,
             CreateAt = DateTime.Today,
             IsBanned = false,
             UnderInvestigation = false
         };
         IdentityResult result = await _userManager.CreateAsync(appuser, user.Password);
         if (result.Succeeded)

            
         return Ok($"User Name {user.Username} was sucessfuly Registered");
         else
         {
             foreach (IdentityError error in result.Errors)
                 return Ok(error.Description);

              //  ModelState.AddModelError("", error.Description);

             return BadRequest(ModelState);
         }

Up to this point my understating is that I don't have to check in the above code whether the ModelState is valid ( just only on the client side right?) . When I run the code in Swagger, all the validation messages pop up e.g Password must have non alphanumeric character, uppercase, if the Username is already taken, etc. All this messages are shown in a response body with a 200 Staus Code.

Back to the client application that consumes the WEB API, I have the following Code

    [HttpPost]
    public async Task<IActionResult> Registration([FromForm] RegisterViewModel model)
    {
        if (ModelState.IsValid)
        {
            var httpClient = _httpClientFactory.CreateClient("HousingWebAPI");
            var body = new StringContent(JsonConvert.SerializeObject(model), Encoding.UTF8, "application/json");
            using (var response = await httpClient.PostAsync("https://localhost:7129/api/Auth/Register?", body))
            {
                /* Checking first if the API is sending a successful StatusCode */
                if (!response.IsSuccessStatusCode)
                {
                    var responseContent = response.Content == null
                        ? "NoContent"
                        : await response.Content.ReadAsStringAsync();
                  
                    TempData["testmsg"] = response;
                    return View("/Views/Registration/Index.cshtml");
                }

                else
                {
                    TempData["testmsg"] = response.Content.ReadAsStringAsync();              
                   
                   return RedirectToAction("Index", "Registration");
                }
            }
        }
        return View("/Views/Registration/Index.cshtml");
        
    }

}

My questions are:

a) How I show to the client via a bootstrap message box all the messages coming from the WEB API?

b) Does the ModelState.Is Valid assures that all the Validations take place in the customer side hence the WEB Api doesn't have to do it?

c) In the event that any developer "forgets"to check the validation, is there any way that the Validation messages from the WEB API can be shown?

d) Is the correct approach to set the response in a TempData and then show it in a Bootstrap message box? if so, how to do it.

Thanking you.

Jose