I've finally succeed. It took more than a month to figure out how is working: with Alex's last suggestions I've reached dotNET node. But even from dotNET node is not easy to get the instance of ItemTextEventArgs, using TestComplete manual (https://support.smartbear.com/testcomplete/docs/app-testing/desktop/net/creating-class-instances.html), which doesn't work with my tested application.
So, what does saved me was TestComplete message error, which was clear stated that it cannot convert the variable I was using to type 'Framework.UI.ItemTextEventArgs'. Later and by chance, I found this instances under dotNET node.
So, this is how the function should look like:
SelectedCellText:=ListViewdObj.GetCell(ListViewdObj.HeaderItems.Item[columnNo],lineNo,columnNo,Sys.Process('YourApplication').AppDomain('YourApplication.exe').dotNET.Framework_UI.ItemTextEventArgs.zctor(),'Excel');
and it works for me(with Delphi script and TestComplete 14.91.341.7 x64).
Now, SelectedCellText will be an object and the cell value will be SelectedCellText.OleValue
Thank you!