All actions: Is an extension of the deadline possible, and if yes, under which circumstances?

This is not possible with the very rare exception of a technical problem caused by the EC/REA. Should this happen, all applicants who are registered in the research participant portal (e.g. by having created a draft proposal) are informed automatically.
It is not possible to extend the deadline for any personal, professional or technical reason an applicant might have. It is the applicant's responsibility to submit the proposal before the deadline of a call in the right format, with all the requirements outlined in the respective call's documentation.