I am a Madrid fan, here is my opinion about this.
There is no need for Gundogan, it is better to go for someone like Pogba/Vidal who do actually fit the ball winner/b2b kind of role. Basically a rich man's khedira.
Im not saying that the rumors are false about Madrid having dibs, but i do think it might be part of a secret agenda from Perez in order to deviate eyes into a new topic. This topic will resurface throughout the year and time and time again barca might be mentioned as losers in the race for gundogan, etc.
Bottom line, i think theres enough reason to believe that this is bullshit. In the sense that there are other reasons behind the sudden outburst of info.
Regarding Illarramendi and Isco, Carlo is a much more "serious" tactician and manager that mourinho is, you can actually have some faith we carlo about how he is going to set everything up. Carlo is very open, during his last press conference he heaped and heaped praise on Illarramendi, and said that isco was coming back from injury but is now ok and part of the competition with modric, obviously he has to be allowed to set in and that will happen imo, no need to jump any ships atm.
Illarra and Isco are 2 of the gems from sub 21 that cost like 70 mil together, they wont be thrown to waste, most importantly they seem to have the coach's approval.
Now i must admit, i dont really watch dortmund, im not completely informed about gundogans capabilities but he seems to me like he could possibly play a b2b role and turn into a a perfect swiss army knife, he seems to be a very complete player, who knows if this could work.