Yeah, emotional affection plays a big part too. I think the 70% happened bcs Muhyeok imprinted on colonel hajung, so even if he's in a different body he subconsciously recognized him. As to why it's still 70%, well, Hajung still needs a lot to adjust, but he's steadily achieving what his og body had (e.g. his class after the re-assessment is just one level below his og body)
As in lovers or friends with benefits? As far as I read, no. The vibe is giving childhood friends BUT it seems Sunghan had feelings. What made Hajun depressed is the survivor guilt, the fact that he couldn't save his guide when he was right in front of his was eating at him. The others can see it's not his fault but he still felt responsible to the point that he wanted to end his life after retirement
I'm still at early chapters of the novel but have a feeling the suspicious guy in the military uniform is his ex-partner who's supposed to be dead while on duty. Oh btw, Hajung's shocked face when they were told the match rate was 70% is more because his pride was hurt lol since their og match rate was above 90%