I heard Juve is going for De Gea and a couple other United players this January. Could this be an opportunity for Ronaldo (and his agent) to engineer 'a return'? 
There is no way Juventus will go for DeGea, they already have a good and cheaper goalkeeper.
To be honest looking here, https://www.transfermarkt.it/manchester-united/startseite/verein/985 they can go only for Pogba or Rashford the others are worse than the other players actually on Juventus.
Yes that is indeed true. Buffon and Wojciech Szczęsny are two great goalkeepers that Juventus have at the moment. So it will not happen, that Juventus wants to go to De Gea.
And also Ronaldo will never return to his past club, he is not yet satisfied for Serie A.