Mitch,
Answering your questions:
1) no, there is no consensus (what did you expect? ja!!). Both Apple and Windows-based systems can achieve very high levels. Maybe a Windows system can be said to allow people to go the furthest by allowing people to switch more parts, but at that point you really need to understand computers.
2) Many examples around. Have you checked computeraudiophile.com? Good source for this. Check out their CAPS 2.0, which are DIY proyects, but can also by purchased from Green Computers. More relevant to your questions, I think, is you can use JRiver for Windows and JRemote that is an application for iPad to use as remote for JRiver. So you can run JRiver in your existing laptop and JRemote in your iPad.
3) I don't see an advantage from this option, especially since you need to buy the MacBook.
4) I think you should focus more on the USB to SPDIF converter than the computer. And the DAC, if your Erato doesn't have an input to the DAC. Nice system, BTW! The USB converter will have async USB and a great clock, so this clock will command the stream, reducing jitter. An Audiophilleo is a good example (I'm considering one) of such converters. Some DACs have converters within, but even many of those benefit from an external source. After DAC and USB converter are dealt with, you will likely hear upgrades at the computer.
Also, ripping music is non trivial - as everything in this hobby! I suggest you follow computeraudiophile's guide to ripping music, that recommends how to set up dBpoweramp. I do this and I'm very happy.
Maybe you get your feet wet using the existing iPad and Windows laptop in the house and getting a USB converter and DAC, and go from there. FWIW, that's the approach I am taking too. I haven't decided between Mac mini and PC for the future, but if I were going Mac I would probably take the Audionirvana+ route (software) to achieve the best sound (bit perfect). Like you, I'm more familiar with the Windows environment, but I'm also enticed by the seamlessness of an iPad/mini interface. Will cross that bridge when I get there. For now my focus is the USB converter.
Sorry for the long message. I hope it helps.