Executing UninstallString in Inno Setup

That’s not a (Inno Setup) constant. That’s a GUID. Remove the ExpandConstant call.

And you need to split the uninstall string to a program path and its parameters.

var
  P: Integer;
  UninstallPath: string;
  UninstallParams: string;
begin
  // ...

  // In case the program path is quoted, because it contains spaces.
  // (it's not in your case, but it can be, in general)
  if Copy(sUnInstallString, 1, 1) = '"' then
  begin
    Delete(sUnInstallString, 1, 1);
    P := Pos('"', sUnInstallString);
  end
    else P := 0;

  if P = 0 then
  begin
    P := Pos(' ', sUnInstallString);
  end;
  UninstallPath := Copy(sUnInstallString, 1, P - 1);
  UninstallParams :=
    TrimLeft(Copy(sUnInstallString, P + 1, Length(sUnInstallString) - P));

  Exec(UninstallPath, UninstallParams, '', SW_SHOW, wWaitUntilTerminated,
       iResultCode);
  // ...
end;