Thank you. The ex-div price date or the date on which the market price falls to the extent of the dividend can be taken.
I dont know why it should take longer with Libre office. It is a simple sheet and should work as is in all spreadsheets.
For portfolio XIRR, you can use the same sheet and simply add all your transactions across stocks in the same sheet. However, there can be some issues here. I think it is better to treat dividends as cash flows since they can be used to buy another stock. So a distinction has to be made between calculating instrument XIRR and investor XIRR. As of now, I have not made this distinction, but it is not hard to make those changes.