I think Messi decided to rejoin Barcelona to play for few more seasons and leave the club amicably with a proper send forth unlike his last unceremoniously exist to PSG which caused some rift between the club and him, of course his entire playing career was spent in Barcelona thus should be rewarded with a club legend status when he finally quit accepting a paycut to rejoin a club that made him a star isn't a bad idea of course other players could be tempted with mouth watering contract offered by the Saudi Arabian clubs, personally I think Messi return to Barcelona wouldn't bear any consequence rather both parties will benefit.
This is an opportunity for Messi to become the highest-paid sportsman in the world. I am not sure this opportunity will be as lucrative as it is now next year. He needs to move now that he is highly needed. Messi is already a legend in Barcelona and he has won everything. His return might be even disastrous because he might not have all the attention he use to have. After all, Robert Lewandowski is now the key player. Maybe he can return to Barca after two years in the SPL league and take up a managerial job.
I am not really sure if Harry Kane is really a candidate for Real Madrid. So far i think this are all just rumors. I mean it totally makes sense that Real Madrid is also thinking about Harry Kane because now that Karim Benzema has left the club they need a new number 9 and they also need it pretty fast. Looking at the market there are only a few players that may have the same level as Benzema and Harry Kane is definitely one of those. He is already 29 years old though and it looks like Tottenham wants a lot of money for him. As a club i would rather spend 100 M€ on a player that is 25 years old than on a player that will be 30 in a few weeks.
Working with one of the best coaches in the world, partnering with some of the best strikers, and receiving the assistance of some of the best midfielders in the world would be a pathway for Harry Kane to win trophies if he moves to Real Madrid. But as you observed, it will be risky to pay a whooping £100m for Harry. No doubt he is a good striker but age is no longer on his side. Harry might be fortunate to move to Real and win trophies like Gareth Bale but it could also be a blunder.