Since Ranamon can't have the help of Rika, I don't think she can digivolve. But she doesn't even need to in this fight. Against other humans with our limited physical abilities, Voldermort is fearful because of his magical ability. But Ranamon could dodge the killing curse that Voldermort casts easily, and can knock him out and finish him without using a special attack. I suppose he may be able to evade during the fight before he is killed, but that still counts as a defeat.
I'm not really sure. Renamon is fast enough to dodge any curse or general offensive attack by Voldermort but she wouldn't dare be able to get close enough to him to attack and any of her long distance stuff can be blocked by charms. Could go either way really.
Legilimency will be a bitch in this fight, not only that but since its a fight to the death, there is no real way for renamon to win seeing as Voldemort has 7 horcruxes. And are people forgetting that Voldemort can apparate? And he was given prep time? And he is bloodlusted? And he can fly(which people also forgot) its not going to be as easy as some may think