Your Rav is correct that most of the time (in fact, nearly all the time), a Shəvah under the first letter of a syllable should be Na', but he is not correct about the Dagesh.
Shtayim is the correct pronunciation, and is even evidenced by the fact that the Dagesh wouldn't be in the Tav if the Shəvah weren't Naḥ.
See, for example, Arabic, which, as a way of compensating for the same phenomenon, has an introductory vowel, an Alif, before the Shin (well, really, its cognate; the Arabic/Hebrew relationship is funny).
Another example to compare with Arabic is Ibn. In Hebrew it's Ben. Why the difference? Because both languages are compensating for the fact that it's really Bn. One (Arabic) compensates with an introductory vowel, while the other (Hebrew) compensates with a change in the first vowel. Yehoshua' Bin Nun can be seen as an example where the original pronunciation was somewhat preserved (this question asks why his name is Bin Nun instead of Ben Nun, and the answers tend to assume a more Midrashic reason, although this one delves a bit into the grammar).
Other examples abound in Arabic, and many Arabic-speakers have a distinct accent when speaking English or other languages with double consonants at the beginning of words, such as "eblease" (please; there's also no 'p' in Arabic), and my favorite example, Eskander, the Arabic name for Alexander ('Al' being treated as the direct object and therefore dropped, leaving a name starting with 'X', which is transposed to 'Sk', which is a double-consonant, which gets the helper-vowel 'ə' appended to it).