@Markoff said:

I haven't understand the point of trying to cancel detonation, wasn't the plan abandon the ship in shuttle and let it explode? why would death of the last 2 crew members change it?

The plan was to get into the shuttle, but the Alien was blocking the corridor / shuttle door, so Ripley instead of charging it with the flamethrower, went for the safer option and tried to stop the countdown. When it did not work, she had no other choice than to get in the shuttle anyway, and by then, the Alien disappeared. We all know where it went of course.