Since Poseidon is DTC, they should be willing to warranty some things that a bike shop would have caught. But I just watched their assembly video, and it didn't say that you should check the stem to make sure the spacers are right. It's already on the bike when the consumer gets it, and they just tell you to loosen it and turn it around the right way and tighten it up again. So having the spacers wrong really is Poseidon's responsibility. That's asking for liability. It's really common for this to be wrong on new bikes. Having an 18mm gap is unusual though, I would say.
It's not too surprising that the fork was really heavy. Aluminum isn't a great material for forks, and they are usually overbuilt. A friend had a suspension fork on a bike and decided to get a rigid fork to save weight. When he finally weighed the two forks, the suspension fork was a little lighter.
Another issue that causes clicking on aluminum forks is they only weld them at the bottom of the steerer. So the steerer is pressed in and welded at the bottom. Lots of them start clicking at the crown race. It's a problem.