I don't see why it isn't plausible that in the absence of a queen, a lone alien could instigate an alternate reproductive process where eggs can be created out of hosts. Its like how some reptiles and amphibians can change their genders in a single-gender environment in order to ensure procreation, a cool survival mechanism. According to Alien, the creatures have a lifespan of only a day or two, which is evolutionary suicide unless every specimen can reproduce on the spot, which in the film is exactly what it starts doing from the moment it matures. It also explains why it would be attacking and carrying off people if there was no queen. So, a queen can be used for mass-producing eggs in a colony/hive environment, or single specimens can reproduce using hosts on a one-to-one basis for pure survival reasons.