It seems like the original text hugely oversimplified the issue. It also relied on the outdated notion of a "fourth-century crisis". I will try to clear a few things up.

Athens is considered to have become a democracy through the reforms of Cleisthenes about 507 BC. Throughout the century that followed, the political system gradually "radicalised", which is to say that the old rich elite were increasingly stripped of their traditional powers while efforts were made to include the general population in the democratic process. Since many of the Athenian poor served as rowers in the fleet while the rich traditionally served as heavy infantry or cavalry, the democracy is sometimes said to have become more "naval" in step with its "radicalisation". The more poor people took part in policy-making, the more focused on naval expansion that policy became. Many Athenian authors, themselves invariably rich men, regarded democracy as "mob rule" by the "naval mob" (nautikos ochlos).

The backlash occurred at the end of the Peloponnesian War. In 411 BC, the democracy was replaced by the oligarchy of the Four Hundred. This was short-lived; a wider oligarchy called the Five Thousand soon took over and restored the democracy. In 404-3 BC, after Athens' defeat in the Peloponnesian War, the democracy was again dissolved and the city fell under the sway of the brutal oligarchy of the Thirty, supported by the wealthy cavalry. Their reign of terror was cut short, however, by a democratic insurgence, and the democracy was restored a second time.

The system of government put in place in 403 BC is the system described by Aristotle (or one of his students) in the Constitution of the Athenians. As a democracy, it was even more radical than its 5th-century predecessor; it introduced revolutionary things like pay for Assembly attendance, allowing even the poorest citizens to take part in the democratic process. It was also one of the most stable political systems in Classical Greece, remaining largely unchanged and unchallenged for over 80 years, until the Macedonian Antipater forcibly dissolved the democracy a final time in 322 BC.

Now, since the 4th century BC saw nothing like the Athenian Empire of the century before it, and the city-states of Greece seemed unable to invest in public works or warfleets on the scale Athens had during the 5th century BC, the period has traditionally been seen as one of decline or even crisis. However, this view is increasingly questioned by scholars. Many Greek states appear to be doing very well in this period. Indeed, more efficient taxation and public finance meant that Athens was approaching the prosperity it had known during the height of its empire - but this time it generated that wealth all by itself. Though its hopes of building a new empire (fuelled no doubt by the "naval mob") were crushed during the costly Social War (357-355 BC), its trade and population were growing, and it may have maintained a fleet on a late-fifth-century scale during the final years of the democracy.

Your original text appears to have conflated two unrelated issues: the radicalisation of the democracy (mostly a 5th-century development) and the "crisis" of the fourth century (mostly an invention of modern scholarship). There is little indication that Athens was becoming more "naval" in the course of the 4th century, or that this caused any kind of crisis. As I said, the political system was stable, and the citizens mostly benefited from the state's economic and military policy.

Plato and Aristotle, meanwhile, were suspicious of democracy primarily because of their own social station. The rich (who paid all the taxes) were rarely in favour of being ruled by the masses (whose wages were often paid by the state). Their deliberations on ideal government fit into a long Athenian tradition of debates over how the political system could be improved.

I believe this answers question 1. Regarding immigrants, they did not have voting rights in any Greek community and only served to make a state more populous and prosperous. Their presence or absence would not have affected the political system.

As for question 2, yes, after the Peloponnesian War the Athenians repeatedly made alliances with other states against Sparta. During the Corinthian War (395-386 BC) they joined together with Corinth, Boeotia and Argos against Sparta. During the Boeotian War (378-371 BC), they were allied with Thebes against Sparta, and also had the full weight of the Second Athenian Confederacy behind them. After this, Spartan power was broken and the balance of power shifted. During the wars of the 360s, Athens actually made an alliance with Sparta against Boeotia. It gets more complicated from there.

As for question 3, the rise of Macedon is a complex story that cannot be explained simply by pointing to the wars of the Greek city-states. No doubt Philip II repeatedly took advantage of the weakness of his southern neighbours, but he also suffered setbacks in his wars against the Greeks, and Athens in particular proved a tenacious opponent. But this might be worth another thread.

I hope this helps. Let me know if anything is still unclear.