With this person's help,
That way, I gained special grace from the shogunate...
As if I could be happy in this remote place!
I accepted a chance to get revenge on the shogunate!
ははははは！！ そうとも！ 儂はまだ徳川の世に納得したわけではない！
Hahahahaha! Not only that! I will never consent to Tokugawa age!
The late start of the warring era....this dream of this country ended....
I'll only accept that!!!
I will let "Death" loose on the shogunate
Who would have thought this man would be so much trouble to dispose of...
Fufu....He's rejecting that wound
I can't do it myself it seems
But "That guy" will drag him along
*I was told that if I did, there would be a special reward from the shogunate
*I think the 安泰だと is referring to the "reward" from the shogunate. I.e. they promised they would keep his castle and lands safe/secure. So you could translate it as, "safe, they said!" "said they would keep them safe!" or something like that.
Bit of a tricky sentence to figure out. Makes it easier if you think その男が相手＝幕府に仇なすモノ. So something like - I accepted an offer from that man's target (opponent, enemy, person he was after etc), someone sworn against the shogunate (out to get revenge on, pitted against, seeking revenge on etc) Does that help? Maybe "enemy of the shogunate" is simplest?
* "I will never" is a bit too strong for まだ I think. "I haven't yet" or "I still haven't" might be closer to the mark.
*how about phrasing 徳川の世 as "Tokugawa's reign" or rule or something similar?
*儂は認めぬ is the same as 儂は認めない... so I won't/can't/will never accept. This makes the line above it a bit clearer as well, so "I'll never accept that we joined the war late or that our dream of stealing the land/usurping the throne/taking over the country..." not quite sure how 国盗りの夢 should be translated, context could help there.
* The "death" unleashed by the enemy of the shogunate.
He was surprisingly/unexpectedly strong/powerful/skilled and hard to dispose of/deal with/take down/defeat.
*退けられまい = 退けないでしょう = 逃げ切れないでしょう, so, "He'll never manage to escape with that wound" i.e. the guy is badly injured, he's never going to make it out. Close to "with a wound like that, he's finished/he doesn't have a hope" etc
*儂は手を下さない -> 手を下すcan be "I won't have a hand in it", so here it seems to be he's saying "I won't be the one to kill him" or "I won't finish him off"
Hope that helps! Let me know if anything isn't clear.