I will be very surprised if Chauvin doesn't get granted a new trial. It may not happen at the first appeal but it probably has to happen once the appeal, if refused, gets to the supreme court.
The problem isn't that Brendan Mitchell attended a BLM rally. It is that he lied about it during voire dire. Had he told the truth he would have been excused without cause. His explanation that it was a march to commemorate MLK doesn't really hold much water, and even if it is the case, the judge should have been made aware of it, and in the interest of justice, he would have excused Mitchell.
The other items in the appeal will be easily overturned. There is one point in the written fillings which is interesting and that is that the judge refused to subpoena Morries Hall, the drug dealer that was in the car with Floyd at the time of his arrest. It's interesting because firstly, Chauvin was not allowed to face his accuser, which doesn't add up to me, Hall was never really material, other than being able to give evidence of George Floyd taking drugs. However, other than Hall's right to not implicate himself in the supply of controlled drugs, I can't see a reason why the subpoena was refused.
Bare in mind that, if the evidence is strong enough, justice will be served by a new trial, untainted by Mitchell's behaviour. I would assume that is the argument Nelson will use.