There are a lot of potential answers to these, and you could probably fill a book describing the state of this election and the causes that led to it. I'll try to explain as best I can based on my perspective and understanding.

Is the American polity changing?

What are the probable causes for it?

Long story short, there's a populist wave sweeping the country fueled by a dwindling middle class that see most current elected officials as not truly representative of their constituents and beholden to moneyed / special interests.

Does this look like a red-herring for the chances of rise of a third-front in future and an end of 2-party politics?

It depends on who gets elected. If we get Trump or Clinton, the existing two party system will largely remain in tact. If we get Sanders and he's able to enact some electoral reform I think you'll see the two party system chipped away to some extent.

However, the two major parties are so entrenched in our political process that it's almost impossible be a prominent politician without throwing in with one of them. By becoming a member of either party you gain access to a wealth of resources (connections, money, etc.)

Is American Conservatism weakening?

I wouldn't characterize it as "weakening"... Its more like an identity crisis. Since Obama was elected the republican party, on a national level, has become fractured and obstructionist.

What does this mean for the immigrant population over there?

Thankfully I don't think any President, by themselves, can have an outsized effect on the legal immigrant population. If Trump wins the White House... it could be a dark time in America for undocumented immigrants.

How and why is he getting the support of a population that is largely educated and politically active, provided with one of the most transparent and competitive electoral processes and has immense sources of information at their disposal?

Trump is only getting the support of Republican voters who vote in the primary process. A lot folks predict that his support would cap very quickly in a general election.