Was the Williams team legally found guilty for Senna's death?

Yes. But nobody paid in the end.

This is very little known but yes, the Williams team was finally found guilty for Senna's death. 
"The Williams team was entagled for many years in a court case with the Italian prosecutors over manslaughter charges, ending in a guilty verdict for Patrick Head. The Italian Court of Appeal, on April 13, 2007, stated the following in the verdict numbered 15050: "It has been determined that the accident was caused by a steering column failure. This failure was caused by badly designed and badly executed modifications. The responsibility of this falls on Patrick Head, culpable of omitted control". Even being found responsible for Senna's accident, Patrick Head wasn't arrested: in Italy the statute of limitation for manslaughter is 7 years and 6 months, and the final verdict was pronounced 13 years after the accident."