I think we'll see Greenwood rotate between striker, right forward and bench as we have a lot of (quality) players to give minutes to instead of running the same ones to the ground. I still think Greenwoods future is at #9 and some of Ole's comments suggest the same.
However, I also think Tony offers something different to both Greenwood and Cavani. The latter has superlative movement and eye for goal, while the former is clinical shooter with both feet. But neither of them has the hold-up play with his back to the goal, or the close ball control that Martial has. If he finds his 19/20 shooting boots he'll be difficult to dislodge from the top, because he can bring other players into the game more than our other #9s. That's a big if though.
In any case it's nice to have 3 entirely different options to choose from and try out in games when one isn't working.
Yeah, I think we'll see lots of rotation this season, we have a lot of quality depth this season. Time will tell if Greenwood's future is up top but I also think he could make the right his own as well, depending on which way he and the squad develop.
You're absolutely right about what Martial offers if he can rediscover his best form, his physicality and technique are quite unique and when he's at his best he can dominate CBs. I reckon he'll get back to that form and like you say, he'll be difficult to dislodge in that case but the competition will be good, keep everyone sharp.
For me, Greenwood has done his best work from the right but has shown glimpses of what he can do up top and certainly gives us a different option from time to time. Over the length of the season, I would imagine the plan is for Martial and Cavani to compete for the CF position mainly.