Acting like Decepticons is not something an organic civilisation can withstand. The pace is too harsh, the demands too severe.

As said, eighteen vorns of Decepticon aggressive harassment. That's about 1500 years Earth time. The Shawsheen are huge and presumably live longer than humans, but that's still going to be half a dozen generations of alien invasions and world-killing plots. Their entire culture had to be slaved to a military complex to keep the Decepticons at bay, but without an opponent, the system turned in on itself.

Yes, the Spearhead is Starscream's flagship [at this time, he got through several over the course of the War].

Of course. He has much to be annoyed about.