THE TRIAL OF HUNTER BIDEN?

The Republican-majority in the Senate can call any witness they want, including Hunter Biden in the impeachment trial of Donald Trump, or any hearing whatsoever,  The question is why hasn't he been called before.  Why hasn't he been investigated by Bill Barr's Justice Department?  Could it be that he already has been investigated and found that he has not violated any U.S. law? There is no law prohibiting any American citizen from working for a Ukrainian company in Ukraine.  There is no U.S. law limiting the amount of money that an American citizen can earn abroad.  Could they arrest him for tax evasion?  They would run into the same laws that are protecting Donal Trump from disclosing his tax returns, wouldn't they?  Perhaps Trump would be happy with the testimony of Hunter Biden but will the Republicans?