It heavily implies that those are the only ways to beat Imu based on Rocks´s statements, but it´s not an outright confirmation. The fact that Oda didn´t have him say anything about training Haki, getting Harald to train his haki, nor his subordinates tells us enough, sometimes the omission of something is more than enough.
Despite what people have headcanon´d themselves into believing, Imu´s vessals have never been stated to be like, 5 tiers below him.
Yes, Luffy is fighting Gunkimu, with the help of Loki, who has one of the 2 fruits.
Nah, The majority of the worstgeneration won´t be surpassing anyone, they´re just the most chaotic. Oda wasn´t even comfortable having Kidd or Law win 1v1´s against Big Mom for a reason.
They can double or even tripple in strength, and they´re not suddenly going to have haki that scales above 5 ACOC users, and even if they did, Oda is only really implying you can fight Imu with special DFs, so it doesn´t even matter.