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Another experiment. 20% of Bitcoin economy that owns 65% of hashing power decided to fork Bitcoin and add unlimited supply of the coins. Let's assume that they would mine 25 BTC per block forever. There are no other differences. What blockchain will a new user download? Of coz the one with unlimited supply because "the longest chain wins". Will the user agree on that? I doubt he will chose 20% of economy instead of 80%. And again we see that economic laws are more important than "mathematical" ones.

A new user will download the longest chain which meets the rules of the protocol. If the longest chain breaks the rules and mines more coins than allowed the blocks are not valid and hence do not count towards the definition of longest.

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If a new user joins Nxt and there are several blockchains he should just ask friends, who brought him there, what blockchain to download (hash of a recent block is enough).

So this scheme achieves consensus via the trusted method of asking a friend. It will probably work but it won't scale.