I think putting Hunter Biden on the stand would be a risky move for the Republicans. Let's look at both the pros and the cons of this move.
Pros (how this could help the Republicans)
1. The logical reason to put Hunter Biden on the stand is to show that asking Ukraine to investigate the Bidens was justified. Donald Trump is the defendant, Hunter Biden could prove that there was corruption in Ukraine.
2. Hunter Biden is a despicable figure; a drug addict who got paid by a corrupt gas company based on his family connection.
3. This paints Joe Biden in a bad light.
Cons (how this could hurt the Republicans)
1. This doesn't logically prove that Trump is innocent and may not even be seen as relevant to the general public (remember the target audience is the non-partisan Americans who by definition don't see things as you do)
2. If Trump is arguing that investigating the Bidens was justified, he is admitting that he was investigating the Bidens. The Democrats can then ask why it was only the Bidens he was investigating (it opens an unconfortable line of questioning.
3. If the general public doesn't buy the Republican narrative about Hunter Biden, there could be a backlash. They could be seen as attacking him only because he is the son of his rival.
I have no idea how Hunter Biden would do on the stand, but if he is well coached (and I assume he would be) he can be contrite about taking the job, and not answer any meaningful questions. He might even get some sympathy from the American public.
Who knows what would happen... this would be a big risky move from the Republicans. Clearly McConnell (who is not an idiot) wants to avoid this spectacle.