WebBrowser control: Navigate shows error message box even though .Silent is true

Question 1

Try handling NavigateError event on the underlying WebBrowser ActiveX control. I have an example showing how handle the "underlying" WebBrowser events like that. It's for WPF, but it can be easily adapter for WinForms (using WebBrowser.ActiveXInstance).

Alternatively, I think you should be able to handle this kind of messages by implementing IDocHostShowUI::ShowMessage. I have another example showing how to implement IDocHostUIHandler on WebBrowser site object. The same approach can be used for IDocHostShowUI.

Disclaimer: I haven't verified either of these two potential solutions.

Question 2

This is a nasty behavior of the WebBrowser control, which I experience with VBA and UserForms. My workaround was to load the target URL into an iframe within a small hosting page. This way no alert pops up, and you can query if the target URL did indeed load or not.

Call the hosting page with <PathTo>/framehost.html#<TargetURL>

<!DOCTYPE html>
<head>
    <title>Frame Host</title>
    <script>

window.onload = function() {
    var frame = document.getElementById('MyFrame');
    frame.src = location.hash.substring(1);
    frame.onload = function() {
        try {
            var doc = frame.contentDocument;
            alert("Loaded " + frame.src);
        } catch(e) {
            alert("Failed to load " + frame.src)
        }
    }
}

    </script>
</head>
<body>
    <iframe id="MyFrame" src="about:blank"></iframe>
</body>
</html>