Days to Expiration, or DTE, is important because it tells you when the option contract will expire. You may choose to exit a position before expiration.
Options only, as stocks do not have an expiration date.
Repeater actions cycle through all open positions. If your logic is to exit at a certain DTE, then yes, you would want to ensure every position is referenced.
If you choose an exact number of "Days to Expiration" and it is a day the market is closed, then the trade will not fill. You will receive a notification that there is no expiration for that date.
Yes, you can always combine criteria together.
It includes weekends and holidays.