Usmanov has no say whatsever in the running at Arsenal. If he did, i doubt Arsenal would be in the position they are in regarding contracts, & he would look to push them into being much more aggressive in the transfer market. It's why there is a power struggle.
As for Kronke forcing a sale, i'm not sure i can see why he needs to. The big amount of money on offer from TV deals help in this situation. Kronke takes his annual dividend out of Arsenal for his KSE group of £3M. That's it, that's what he takes out. So, he won't be concerned about pocketing £50M from a Sanchez sale. He puts no money into the club. He is just sitting on an asset that is increasing in value year after year. As long as Wenger continues to work under the guise of we spend what we make, then the only thing rejecting £50M will do is that it's not in the bank for a potential replacement. However, IF Arsenal can afford £50M for Lemar without selling Sanchez this summer, the likelyhood is that they will have a similar budget next summer for more signings. Ozil is more likely to sign a new deal than Sanchez. Ox isn't a guaranteed starter anyway & it looks like he might need to compete with Bellerin for a place.