Just to touch on some of the OP's questions, from my own personal experience.
All EU citizens may apply for a C permit automatically after 5 years. While the process for B or C permit application may seem automatic for EU citizens, you can actually be turned down if you fail on one of the criteria.
However, there is one caveat about all EU citizens ; certainly this is the case for old (Western) EU citizens, but especially since the initiative last year it may not be as automatic for some (I think there was something about this affecting Croatians).
The Quellensteuer in theory stops after five years regardless of what permit you're on, AFAIK. However, moving onto a C permit means that the Steueramt will be informed of you and you'll formally appear on their radar. So you'll get mail from them shortly after you get your C permit.
No. At least I didn't have to bother with interviews or police checks. All I was asked to get was a letter stating there are no financial judgement against me in X time period, another letter from the social welfare saying I have gotten nothing from them in X time period and my work contract.
Of course I have dual nationality and so registered as an Italian when I first came here; had I registered as Irish I would also have been asked to supply an A2 German language certificate too. Some countries, such as Italy, Germany and France are exempt from this.
I'd double check on that non-Swiss income tax exception if I were you - I'd be skeptical that any country would be so generous.
As for advantages of a C, I believe there are some, such as being able to buy Swiss property or voting rights in some cantons, but in my experience the advantages are only really social and political.
Political in the sense that if they do tighten up things following last years initiative, those with C status will be the least effected (as we're too close to becoming full citizens and thus voters the SVP doesn't want to piss off). Social in the sense that I've noticed you get a more friendly, positive response from some Swiss once you're C status - a permanent resident; here to stay, a permanent part of the community and not one of these blow-ins who comes here for a job for a few years and then blows-out again.