nummer is a bit of a false friend. In Swedish, it refers only to a 'nominal number', that is, a number used for identification. So number 3 is nummer 3, but a number of people is ett antal människor and can never be translated with nummer.
Another example where nummer is used is in phone numbers, telefonnummer, those are also used as identifiers rather than quantifiers.
You can't add av. The construction without a preposition is the normal way, like en kopp kaffe = 'a cup of coffee'. When av is used, it is more for materials, like ett hus av sten, 'a house made of stone' (stenhus is a more frequent way of saying it) or something that is meant to resemble something, like en bild av en blomma 'a picture of a flower'.
Yeah. I believe he understands that in the collective Swedish mind, both the sounds he describes are treated that is one sound. He just wants to know which one to use.
While both ways are absolutely correct. What most would consider standarad is a very well defined pattern that roughly looks like this: Before a vowel it's usually 'hw' like "skepp" After a vowel it's usually what what sounds LIKE 'sh' (but isn't really). Like in "garage" (the final 'ge' sounds like that 'sh')