As promising as the recent info is, the "believe it when I see it" philosophy is still the safest. Lest we forget how many times we've read "ULTIMAS HORAS" this summer in a number of different transfer cases. Board could still easily fuck this up.
As for the lineup we use if/when he's here, honestly I'd love to see how Coutinho and Iniesta play together. I understand that for the most part he'd be playing with Rakitic so Iniesta can rest for the CL, but there's plenty of rotation to see how our two best mids would coexist. I could see them both adapting enough for it to be possible. Iniesta would honestly be fine as a RCM, we've seen him playmake in those areas without difficulty. He would also be fine playing in a bit more of a conservative role since Coutinho can take a LOT of the creative burdens off of him. We saw it a lot with Neymar too where he'd be more content circulating the ball in deeper zones and letting Ney attack hard than trying iso-final pass plays to force chances when our only other creative player is Messi. All in all our ball movement would look much better and Coutinho would help Iniesta/Messi perform to their best.
If we do get Coutinho in January, we should (obviously) be selling Gomes, Rafinha, and Arda without any hesitation at all. Beyond that I'd sell Rakitic for Arthur as we'd need a capable deep lying CM that could possibly start in most easier games so Iniesta isn't overloaded in what will be his Xavi 14/15 season. Aleña should also be promoted to take up the last of the minutes available.
Coutinho/Iniesta/Arthur/Paulinho/Aleña/Busi/Roberto would FINALLY be a midfield rotation worthy of this club and comparable to the best sides in Europe. We'd have all bases covered and no lack of options to control the midfield and attack the opposition.
Edit: forgot about Denis, but honestly I'd have him be a LW/RW sub for Dembele and/or Messi. In the scenario listed above we'd be more than covered in midfield and Denis has the speed, 1v1 capability, and creativity to do well as a winger.