Inno Setup - How to display 3 options: full | upgrade | uninstall? - upgrade

Question

I have a working setup.exe that runs 'full' and 'upgrade' installations from a proxy server that elevates the user to admin only during execution of setup.exe.
While the setup.exe successfully creates an uninstall icon, the user does not have sufficient privileges to execute an uninstall. :(
So, it is desirable and efficient for me to submit a single setup.exe to the proxy server that offers the users a single point of reference.
How can I add a 3rd selection to 'uninstall'?
My initial thoughts are to trick the [Types] section:
[Types]
Name: "full"; Description: "Full Install"
Name: "upg"; Description: "Upgrade Only"
Name: "del"; Description: "Uninstall"
But which event do I code in to start the uninstall process?
Is InitializeSetup() too early? Meaning the user hasn't selected anything yet.
EDIT: After much searching and prompts from TLama, I have something working.
// code snippet...
Function GetUninstallString(): String;
var
sUnInstPath: String;
sUnInstallString: String;
begin
sUnInstPath := ExpandConstant('Software\Microsoft\Windows\CurrentVersion\Uninstall\{#emit SetupSetting("AppId")}_is1');
sUnInstallString := '';
if not RegQueryStringValue(HKLM, sUnInstPath, 'UninstallString', sUnInstallString) then
RegQueryStringValue(HKCU, sUnInstPath, 'UninstallString', sUnInstallString);
Result := sUnInstallString;
end;
Function Do_UnInstall(sUninstallString: String): Integer;
var
ri: Integer;
begin
// Return Values:
// 0 -> uninstall string = empty, new install?
// 1 -> successfully executed UnInstallString
// -1 -> error executing UnInstallString
Result := 0; // Default value
if sUnInstallString <> '' then begin
sUnInstallString := RemoveQuotes(sUnInstallString);
if Exec(sUnInstallString, '/SILENT /NORESTART /SUPPRESSMSGBOXES','', SW_HIDE, ewWaitUntilTerminated, ri) then
Result := 1
else
Result := -1;
end else;
end;
Function NextButtonClick(CurPageID: Integer): Boolean;
begin
Result := True;
case CurPageID of
wpSelectComponents:
begin
if WizardSetupType(False) = 'del' then // Uninstall and exit!
begin
OK2Unins := True
if Do_UnInstall(sUnins) = 1 then
Result := False;
//ExitProcess(0); This fails. Prompts user with a cancel message.
WizardForm.Close; // This trigger CancelButtonClick() code...
end;
end;
wpSelectTasks:
begin
end;
wpReady:
begin
end;
end;
end;
Procedure CancelButtonClick(CurPageID: Integer; var Cancel, Confirm: Boolean);
// Predefined Wizard page identifiers
// wpWelcome = 1;
// wpLicense = 2;
// wpPassword = 3;
// wpInfoBefore = 4;
// wpUserInfo = 5;
// wpSelectDir = 6;
// wpSelectComponents = 7;
// wpSelectProgramGroup = 8;
// wpSelectTasks = 9;
// wpReady = 10;
// wpPreparing = 11;
// wpInstalling = 12;
// wpInfoAfter = 13;
// wpFinished = 14;
begin
if CurPageID = wpSelectComponents then
Confirm := not OK2Unins;
end;