I love filter questions.
1--Yes. Tiffen filters are generally excellent. Their standard line of filters don't have the cache of the German super glass multi-coated brands but they do as good a job for a lot less money.
2--Not usually. Multi-coating can help prevent or reduce ghost images if you shoot pictures against the light with a filter over the lens. It's usually safer and cheaper just to take off the filter.
3--Maybe, maybe not. Hoya and Tiffen both make filters that are sold as "house brands", I've read. Their filters are good. If "generic" means "third rate" wavey glass mounted crooked in the filter ring, oh yeah, you'll be able to tell the difference.
4--Just about anyone who has been making filters for a long time know how to make optically clear and flat glass (or even plastic, in some cases). I have filters made by Hoya, Tiffen, B&W, Nikon, Canon, Heliopan, Singh-Ray, Vivitar and even a couple of old Spiratone filters. I can recommend all these brands because all of them are of excellent quality. In fact, the only brand of filter I've ever used that I would never recommend is Cokin's plastic "system filters". *See below.
5--If 58mm is the size your lens takes, it's what fits. But if you intend to buy other lenses with different size filter rings, you will come out cheaper in the long run by buying filters to fit the lens with the largest size filter thread and buying step-up rings to attach them to the smaller sizes. It will prevent you from using Canon's bayonet mount lens hoods, however. If you want to use the Canon lens hoods, you'll need filters that fit every lenses threads directly.
*Once upon a time, I bought a number of Cokin filters so I could save money and fit a lot of different brand and sized lenses. When I tried to use them on a 400mm Canon lens, I couldn't even see through the lens. Upon testing the filters with other lenses, I found that there was detectable distortion with every focal length I owned of 100mm or longer. Some Canon lenses would not even autofocus with Cokin filters attached. Using normal or wide angle lenses, the distortion could not be detected--attributable to the magnification factors of the lenses involved. But having a 100% failure rate with something like 10-12 Cokins, I came to the conclusion these filters are simply not very good.