Firstly from Gaban himself and no alternative Loki just giving himself up to shanks does not align up with how Oda writes shanks.
Why are we leaving out the fact that Harald stopped fighting and allowed Loki a free hit to Finnish him off
Gaban did not once said that Garp and Primebeard could not do the same. In fact Gaban was surprised that Harald could use conquers haki and did not know you can overcome it by using mass which something Gaban and Shanks never tried to do.
From the spoilers Loki managed push Imu away ok it didn't injure imu at all, we saw Garp and Roger do the same with DR Xebec the issue was overcoming the conquers haki defense. Also Imu isn't going all out with Loki its obvious he's playing around with him, this is normal villain writing they never go all out from the start.
Lastly scars are a narrative tool used by Oda they are never used for damage from rights, Garp and Roger got no scars from the xebec fight hell Luffy died against Kaido and he doesn't have a single scar from it. Are those fights not battles at all now as well?