Would German-speaking people notice the difference of pronunciation between iShelf and iSelf?
Yes, they would.
iShelf is pronounced [aɪʃɛlf] and iSelf is pronounced [aɪsɛlf]. The differing sounds [ʃ] and [s] are both part of the German phoneme repertoire and distinguishing them is essential to understand the German language. For example, the German words Busch ([bʊʃ]) and Bus ([bʊs]) or Sex ([sɛks]) and Schecks ([ʃɛks]) only differ by that sound. Thus German ears are trained to that difference and will notice it.
By contrast, Germans usually have problems in recognising the difference between man ([mæn]) and men ([mɛn]), as [æ] is not part of the German phoneme repertoire.
Absolutely. [s] and [ʃ] represent different phonemes in German (just as in English), so speakers of German will distinguish them.