Lets me star by indicating that I am not a pro so if you want to stop reading further feel free. I waited for a pro to respond before writing my post. Also keep in mind that for every DIY issue there may be more than one way to go about solving a problem.
Here is my take. You are right about sistering additional joists. That is what I would do. I would also jack up the sagging floor so it is level. If you are really lucky the floor leveling compond will crack and can be removed by hitting it with a sledge and using a pneumatic chisel. After all if you level the subfloor you won't need the leveling compound anyway. I haven't worked with floor leveler so I can't predict much except that you can expect it to crack. If you want to keep the leveling compound you could rent a device that looks like a floor buffer but with diamond abrasive. Regarding the use of hangers, if the joists rest on a sill plate then the new ones would not need hangers. And since you are sistering, you probably would not need them anyway. If you do go for hangers (should not have negative ramifications should you go that route even though not necessary; think wearing a belt with suspenders) get hangers that will hold a double joist so the sister will be in direct contact
with the faulty joist. I use screws and adhesive to join the joists but nails can be used as well.
Here is my take. You are right about sistering additional joists. That is what I would do. I would also jack up the sagging floor so it is level. If you are really lucky the floor leveling compond will crack and can be removed by hitting it with a sledge and using a pneumatic chisel. After all if you level the subfloor you won't need the leveling compound anyway. I haven't worked with floor leveler so I can't predict much except that you can expect it to crack. If you want to keep the leveling compound you could rent a device that looks like a floor buffer but with diamond abrasive. Regarding the use of hangers, if the joists rest on a sill plate then the new ones would not need hangers. And since you are sistering, you probably would not need them anyway. If you do go for hangers (should not have negative ramifications should you go that route even though not necessary; think wearing a belt with suspenders) get hangers that will hold a double joist so the sister will be in direct contact
with the faulty joist. I use screws and adhesive to join the joists but nails can be used as well.