The sentence:
- As it turns out your assuptions were true, as has been evidenced by her conviction for murder last week.
is correct. Alternatively, "was" can also be used:
- As it turns out your assuptions were true, as was evidenced by her conviction for murder last week.
Or, still better, we could use a reduced participial clause:
- As it turns out your assuptions were true, as evidenced by her conviction for murder last week.
The reason why the present perfect is possible despite the presence of the adverbial "last week" is that this adverbial may be considered as modifying "her conviction for murder":
- As it turns out your assuptions were true, as has been evidenced by the fact that she was convicted for murder last week.
What happened last week was that she was convicted for murder. The realization or the recognition that the interlocutor's assumptions were true is not time-dependent and may be more recent, and this renders the present perfect perfectly possible.