.... Since I have not heard back from him so far ..... Yes
.... Since I have not heard back from him yet ..... Yes, though this is better
.... as yet we have received 25 complaints ..... No. As yet at the beginning of a sentence is normally followed by a negative - "As yet, I haven't spent any money, but I will later.
.... so far, we have received 25 complaints ..... Yes
See above. With your first example, I assume the full sentence would be something like:
Since I haven't heard back from him yet, I'm going to have to email him again!