Can it turn out that despite the ticket and check-in, I will not receive a seat and will not fly in the regular time?

Yes. Most often this is because the airline has sold more tickets than the capacity of the aircraft, all passengers have checked in and reported to the Gate. In such a situation, the airline is obliged to offer the passenger an alternative connection to the destination airport at its own expense and to pay compensation for refusal to fly. The passenger should receive a leaflet about his rights in such a situation, the so-called passenger rights. Everyone can also access them via the carrier's website. If you know that volunteers are looking for volunteers on the cruise, and you can afford a later cruise, apply! This is a good chance to repair your own funds ;)