when they scan your passport they will automatically know whether TDAC is filled or not. I saw many people being turned away from immigration after passport scanning because TDAC was not filled.
I have put departure as the date when I am leaving Thailand and have faced no issues while entering Thailand. Arrival and departure are both related to Thailand. If it is long term stay and your departure is not fixed then leave those fields blank.
Without return ticket Indians don't get either VOA or E-VISA. You can buy dummy tickets and overstay though but that would mean a permanent marker in your passport and problem while visiting Thailand in future.