Can McConnell refuse to seat Nelson if fraud succeeds to putting him ahead and force another election
Yes. The Senate is the final arbiter of its members (however, some pissant Derp State judge will butt in to illegally and unconstitutionally “order” the GOP Senate to seat rodent creep Nelson).
According to the Consitution, each House is the judge of their own returns, so if it goes on that long the entire Senate could take up the issue.