I thought that in history, Shin as the one who proposed to only send 200,000 men despite Ou Sen stating that something like 600,000 would be required, but Sei decided to trust Shin and sent him to conquer Chu.

Also, I don't remember reading that any traitors were involved. Shins Army was ambushed upon moving into Chu lands, and were able to push Shin back due to the intelligence of the commanding General in charge of the Chu armies.

No, personally I don't have much heart for Sei. The saddest part will be in the arc when most of hi shin unit will be killed by traitors after Shin started the campaign to conquer Chu but ironically Ousen takeover the campaign (Ousen refused the campaign because Sei didn't provide him with enough number of troops, but on the other hand Shin accepted the proposal that was sent by Sei even with small troops, note here Ousen will have short retirement when Shin started the campaign but later on comeback and hit it successfully)

By saddest part do you mean after Kan Ki (or whoever betrays Qin as I think I saw somewhere that they historically don't know if he was the one who actually betrayed Qin) betrays Qin and sends assassin after assassin to try and kill Sei and eventually even tries himself but is subsequently killed. However, Sei is now absolutely fucking insane and obsessed with immortality?

I won't spoil it, you'll regret if you know the story because saddest part will be in that arc. (If Hara is still going with the history record tho)

Didn't think he would. He seems to be destined for something... bigger. I haven't really read on the history, so this doesn't really count as a spoiler at all... but does Ousen (or Kanki, for that matter) become a villain later?

Ousen won't die.