diff --git a/WitcherScriptMerger/Forms/MainForm.cs b/WitcherScriptMerger/Forms/MainForm.cs
index 8897f40..06a9cb9 100644
--- a/WitcherScriptMerger/Forms/MainForm.cs
+++ b/WitcherScriptMerger/Forms/MainForm.cs
@@ -882,18 +882,19 @@ void HideProgressScreen()
 		public DialogResult ShowMessage(string text,
 			string title = "",
 			MessageBoxButtons buttons = MessageBoxButtons.OK,
-			MessageBoxIcon icon = MessageBoxIcon.None)
+			MessageBoxIcon icon = MessageBoxIcon.None,
+			MessageBoxDefaultButton defaultButton = MessageBoxDefaultButton.Button1)
 		{
 			this.ActivateSafely();
 
 			if (this.InvokeRequired)
 			{
 				return (DialogResult)this.Invoke(new Func<DialogResult>(
-					() => { return MessageBox.Show(this, text, title, buttons, icon); }));
+					() => { return MessageBox.Show(this, text, title, buttons, icon, defaultButton); }));
 			}
 			else
 			{
-				return MessageBox.Show(this, text, title, buttons, icon);
+				return MessageBox.Show(this, text, title, buttons, icon, defaultButton);
 			}
 		}
 
diff --git a/WitcherScriptMerger/HeadlessMergeNotifier.cs b/WitcherScriptMerger/HeadlessMergeNotifier.cs
index 8212f24..a76ce0b 100644
--- a/WitcherScriptMerger/HeadlessMergeNotifier.cs
+++ b/WitcherScriptMerger/HeadlessMergeNotifier.cs
@@ -14,10 +14,19 @@ class HeadlessMergeNotifier : IMergeNotifier
 		public DialogResult ShowMessage(string text,
 			string title = "",
 			MessageBoxButtons buttons = MessageBoxButtons.OK,
-			MessageBoxIcon icon = MessageBoxIcon.None)
+			MessageBoxIcon icon = MessageBoxIcon.None,
+			MessageBoxDefaultButton defaultButton = MessageBoxDefaultButton.Button1)
 		{
 			Write(text, title, icon);
 
+			// defaultButton is intentionally unused here: it only affects which
+			// button has interactive UI focus (Enter-key behavior), and headless
+			// mode has no UI to focus. The DialogResult below is chosen per
+			// buttons set instead, per this class's own fixed, non-destructive
+			// default for each verb - not by the caller's requested default
+			// button. A call site relying on defaultButton to signal "this is the
+			// safe answer" for a MessageBoxButtons combination not covered below
+			// will get whatever the catch-all case returns instead.
 			return buttons switch
 			{
 				MessageBoxButtons.OK => DialogResult.OK,
diff --git a/WitcherScriptMerger/IMergeNotifier.cs b/WitcherScriptMerger/IMergeNotifier.cs
index c66d0dd..0eb3034 100644
--- a/WitcherScriptMerger/IMergeNotifier.cs
+++ b/WitcherScriptMerger/IMergeNotifier.cs
@@ -9,7 +9,8 @@ interface IMergeNotifier
 		DialogResult ShowMessage(string text,
 			string title = "",
 			MessageBoxButtons buttons = MessageBoxButtons.OK,
-			MessageBoxIcon icon = MessageBoxIcon.None);
+			MessageBoxIcon icon = MessageBoxIcon.None,
+			MessageBoxDefaultButton defaultButton = MessageBoxDefaultButton.Button1);
 
 		DialogResult ShowError(string text, string title = "Error");
 
diff --git a/WitcherScriptMerger/LoadOrder/LoadOrderValidator.cs b/WitcherScriptMerger/LoadOrder/LoadOrderValidator.cs
index 29e88b5..d99f184 100644
--- a/WitcherScriptMerger/LoadOrder/LoadOrderValidator.cs
+++ b/WitcherScriptMerger/LoadOrder/LoadOrderValidator.cs
@@ -21,8 +21,11 @@ public static void ValidateAndFix(CustomLoadOrder loadOrder)
 			{
 				PrioritizeMergedMod(loadOrder, mergedMod);
 			}
-			else if (choice == DialogResult.Cancel)  // Never
+			else if (choice == DialogResult.Cancel && Program.Notifier.IsInteractive)  // Never
 			{
+				// IsInteractive guard: HeadlessMergeNotifier's fixed default for
+				// YesNoCancel is Cancel, which would otherwise persist this setting
+				// on every headless run that reaches here.
 				Program.Settings.Set("ValidateCustomLoadOrder", false);
 				Program.Settings.Save();
 			}
@@ -30,10 +33,20 @@ public static void ValidateAndFix(CustomLoadOrder loadOrder)
 
 		static DialogResult PromptToPrioritizeMergedMod(string modsSettingsPath)
 		{
-			MessageBoxManager.Cancel = "Ne&ver";
-			MessageBoxManager.Register();
-
-			var choice = MessageBox.Show(
+			// Known, accepted regression: the Cancel button used to be relabeled
+			// "Ne&ver" via MessageBoxManager.Register()/Unregister(), which worked
+			// only because the old MessageBox.Show call ran on the same background
+			// thread (Register()'s SetWindowsHookEx is thread-affine) as this
+			// method. Program.Notifier.ShowMessage (MainForm.ShowMessage) Invokes
+			// the actual MessageBox.Show onto the UI thread, so that hook can no
+			// longer see the dialog's window messages - relabeling can't be
+			// preserved without adding custom button-text support to
+			// IMergeNotifier, which is out of scope here. The Cancel button now
+			// reads "Cancel"; clicking it still permanently disables this check
+			// (see the IsInteractive-guarded branch above), just without a label
+			// saying so. DialogResult semantics and this method's return value are
+			// otherwise unchanged.
+			return Program.Notifier.ShowMessage(
 				$"{modsSettingsPath}\n\n" +
 				"Detected custom load order in the file above, and merged files aren't configured to load first.\n\n" +
 				"Would you like Script Merger to modify your custom load order so that your merged files have top priority?",
@@ -41,9 +54,6 @@ static DialogResult PromptToPrioritizeMergedMod(string modsSettingsPath)
 				MessageBoxButtons.YesNoCancel,
 				MessageBoxIcon.Exclamation,
 				MessageBoxDefaultButton.Button2);
-
-			MessageBoxManager.Unregister();
-			return choice;
 		}
 
 		static void PrioritizeMergedMod(CustomLoadOrder loadOrder, ModLoadSetting mergedModSetting)