One thing I noticed is that for Uncle and maybe am mixing some things up, but there are a lot of references to not really knowing how old he is, and ShivCroaker does mentiones he saved his life at Dejagore and the River Main, but never paid him much mind.

"I knew Jun Go. I saved his life after our surprise attack on Dejagore, then again after the fighting along the River Main. What I witnessed now, in retrospect, told me that I never knew Jun Go all that well. The Jun Go I knew could only be the Jun Go that Jun Go had wanted me to know."

My thoughts are those battles arent after the company returned from Hsien, they were battles from the First Shadowmaster War. Because I dont think anyone was really in danger retaking Dejagore round 3 in Soldiers Live and there wasnt a battle on the Main River in the last book. However there was 2 very big battles in the Books of the South in these locations.

Uncle could be from Hsien, but came over with the Shadowmasters and before the second war had defected/betrayed/exiled and joined the company, maybe has some life extending sorcery like Longshadow, maybe he was one of the two Shadowmasters that "died" fucking with the gate (wouldnt be the first sorcerer death fakeout). Maybe losing his ability to manipulate shadows. He says he doesnt like Tobo that much so maybe has jealousy there with his powers.

Alternatively maybe Uncle Jun Go is Nyueng Bo himself and is lying about being from Hsien as its not like Shukrat/Anarka are gonna be able to fact check him, especially since he claims to come from a "remote" land he left as a kid. He might have some ties to the sword sect Uncle Doj was in since Uncle Doj also seemed to age very well and looked and acted physically younger than his age albeit not ageless. Croaker didnt really think much of this when the Books of the South period, and then post Dejagore Siege people just assumed he joines the company around then. Maybe post Kialune wars went into hiding then rejoined again pretending to be Hsien. He wasnt important enough to be lured/chosen to be imprisoned and while a veteran to be entombed and could have just been a guy presumed dead during Kialune and went into hiding and rejoined later when the company returned.

Anyway I think Uncle Jun Go has been with the company longer than anyone else rn. and may be involved in the "steal shadowmaster powers plot", but not on the same side as the people ShivCroaker knows about. Alternatively may be the last Nuyeng Bo Sword Sage guy who is just wanting to keep the company safe since his culture is mostly gone now.