I would say this, and this is plain guessing obviously.
First of all, I didn't see any indicators, none that Messi would go to psg or man city, bar "neymar is at psg too" which is ironic since he run away from Messi, and also "Pep is at City" which they didn't get that along when working together. Now lets look at us:
1) That Suning poster with Messi. Now say what you want, but that was kinda enigmatic thing to do. They didn't do it with anybody else, bar Tonalli who is ours (as indicated) for months now. Just for the fucks of it? Also, they did it long before Messi leaving started to get rounds.
2) Apartments in Milano. Yeah, famous people buy apartments in other cities, but this time it wasn't in Moscow, or New York, or Madrid, or London, or Paris, or anywhere else. It was in Milano.
3) Suning talking about great things from the start. Usually, new owners say a lot of things like that but they do not materialize. Now, I did think it's bs for a while, but I stopped after we got Marotta and Conte. From that point on, I've treated it as meaning business. Cleaning up the house and building a team. Lukaku, Eriksen, now Hakimi only proved that. Say what you want, but things are happening around Inter. Now... what if this time around the owner was not bullshiting, and they did say "Inter is coming". Maybe it is?
4) Conte staying... Messi leaving. Accident? Another "strange" thing. Why is it that after Inter meeting at high level, talking strategies for next year/years to come, immidiately Messi sends his message. Maybe he got the call from his new employer, "green, green".
5) Messi to Inter, many cordial comments here and there over the years. Now this might be just a sign of politness, but those messages between Inter (Moratti) and Messi were circulating from time to time.
6) Messi knows his new employer already. Now if his intention to leave is true, and not some fucked up plot to fire whole barcelona management and then stay, then no fucking way he did all of that firestorm this evening without knowing and having everything set up with his new employer. You don't do that and stand with your dick hanging waiting what's going to happen next. In that case, first of all, you have much weaker hand to negotiate best terms with new club (you've already told us you're leaving) and second, what if you won't find a new deal with the team or two you hope for. Then what? Sign with Lazio? And I doubt he did that overnight, I think if he is honest in his willingness to leave, that deal was already done weeks if not months ago. You don't set up the deal involving milions of dolars and dozens of sponsors etc in a day.
Now, my conspiracy theory goes like this:
Inter made approach to Messi, to which Messi said "Ok, I'm ready". Conte gets to know about it and he hates it for whatever reason. Maybe he doesn't see a place for him, maybe he hates that it wasn't his idea, maybe he doesn't want to break Lukaku Martinez duo, doesn't matter. What matters is that Conte gets pissed (due to some other things too). Messi deal gets done in principle (Messi will ask to leave, Inter will offer him agreed deal) and Conte finds out about it and does not want to continue at Inter. He lets his frustration vent after the season and says we need to discuss things for the future of Inter (read, how to approach the guy I did not want in the first place). The meeting is done among the heighest ranking people at Inter to convince Conte that despite him not being totally on board with getting Messi, he gets convinced that it is a move that will put Inter back on the map of the biggest and gets convinced to stay, but as a reward no expectations are put on him.
Just guessing and it's fun guessing, but who knows. One thing that is great this time is that this thing will move fast. It's not months, starting from may and ending 31 of August. A few days, a week tops and it will be done. We'll see. If it will happen, then it will be great having a player everybody will be talking about, and I hope Conte will not use it as an excuse in every interview, and it it does not, then we continue building our great team. Either way I will be happy.