Aren't Mercedes still rumoured to be considering selling the team in a couple of years, following the loss of Petronas? They might want a couple of decent but inexpensive drivers contracted for a couple of years if so. If they can get Ocon on the relative cheap he might tick that box.
What are you basing this off exactly, they signed a multi year extension to the existing deal with mercedes this year... and the last deal was stated to be a 10 year deal from 2014... meaning they just agreed to stay the course for at least a few years more post 2024 as far as I can tell. Singapore wasn't the reason Petronas were in F1 with Mercedes, they are in F1 for the advertising and relationship with Mercedes.
There is very little hint of Mercedes leaving or even considering leaving beyond Jordan saying so.
It terms of TR seats, Matsushita's test/free practice session supposedly went poorly and he was considered miles slower and no where near good enough for F1 which I think is pretty much why the talk of him has disappeared. Honda aren't interested in having a Japanese driver come in and embarrass them by being terrible. Hell, it would look better for Kvyat to be terrible than a Japanese driver. Kvyat if he finds his form is a very capable driver, but he's been crapped on so much that they killed his confidence completely it seems. There are other options though, loan Wehrlein would be good for them and Mercedes imo. Frankly regardless of it Sauber decide to ditch him or Ericsson, it would be better for Wehrlein and for Mercedes to have him for a year in the TR seat, test him more properly and know if they should back him for the future. You just aren't learning enough having a guy in a backmarker team.
The only thing we really know is that Wehrlein is better than Ericsson because Sauber suck too badly to do anything interesting in that car. Even with the current years Ferrari I don't see them suddenly being even half way competitive. I think Leclerc looks really exciting and it will be a shame to see him in a Sauber looking extremely unexciting because he hasn't got a car that can do anything.
It seems a real shame that Haas seems to be committed to sticking with Grosjean and Magnussen as they turned down the opportunity to take on Leclerc, which would have helped them with money and more help from Ferrari. Sauber may want to do better but I think we all know they won't any time soon, Leclerc in a Sauber seems like a huge let down in terms of exciting new drivers coming in to make things more interesting.
Haas, no idea what they are doing, personally think Grosjean and Magnussen are dead weight making refusing Leclerc even more crazy. They could have gotten Ferrari to buy out either of their remaining contracts to get Leclerc, win win, probably a couple years with Leclerc which could make them a good future team for other young drivers coming through. Sauber is the current young drivers go to kill their career team because the team has gone nowhere in years. Haas had the chance to be the team that accept Merc/Ferrari/Mclaren young drivers and keep constant young talented drivers coming through. Grosjean for me is a bad driver, Monza in qualifying, the only guy to bin it complains about the conditions but was from memory going 30kph faster than Hamilton or Vettel down the straight where he binned it. If rain shows the best and worst drivers, Grosjean is one of the worst on the grid, he screws up literally every wet session or race because he's inept. Even when dry he's only fast because he's reckless and gets into some kinda contact every other race even if it's minor, while also being a whiny ***** who blames the team for everything. It was finally called out earlier this year by the team boss that Magnussen works around the car better and has less brake problems. In other words a huge part of the reason Grosjean keeps having brake problems is because he stamps on the brakes, drives how he wishes his perfect car could be driven rather than taking care on the brakes and driving as the car requires. same deal with Monza, he drove as fast as he could, not what the car could do in those conditions which literally every single other driver managed to do on the day.