I think the only one complicating things is you. To answer your questions:
It certainly costs Apple less to produce the 5c. Plastic is cheaper than metal and glass, meanwhile the costs of the A6 chip would have dropped by now and the A5 stopped dropping in cost a long time ago.
I think you're trying too hard on the accounting cost one.... skipping it. Obviously questions Apple could answer.
A 5c would naturally be more appealing than a 4s. Newer, better performing, larger screen, etc. Was this a serious question?
The impact on Apple's high end models is negligible because no one in that market would consider an 8 GB phone.
No, the answers wouldn't change next month. However, they will continue to swing more and more in the 5c's favour. Following the discontinuation of the iPad 2, the only product that shares the same processor generation with the 4s is the Apple TV, which is due for an upgrade.
If the startup costs of the 5c haven't been written off it could, on paper at least, cost Apple more to produce than the 4S. The fact that the new 5c is priced $50 above the 4S lends weight to that argument. I would say today's introduction of the iPad 4 in place of the iPad 2 (something that could have happened the day the iPad Air was introduced) is further proof that it takes time for initial costs to be written down to the point where the newer product finally costs less to produce than an older one.
Appeal is a serious consideration. The 4S was a flagship phone whereas the 5c has never been anything better than second class. That's why 5c sales are so low in China. Nobody there who can afford an iPhone wants to be seen with a second tier product.
I was specifically thinking about cannibalization of 16GB iPhone 5c sales and the assumption that the price differential would be $100 rather than the $50 it turned out to be. Certainly some will now opt for $50 in their pocket instead of an extra 8GB of storage, but Apple is counting on the lower price attracting customers who would otherwise have purchased a different brand of phone.