I see some criticism in this film for the portrayal of two things that I think is unfair:

_i) how a physically and technically inferior older brother who is carrying injuries/fatigue could defeat his younger, fully fit brother. _

Older siblings, particularly brothers often seem to have an Indian sign on the younger brother, like there is some mental barrier for the younger to overcome someone that was always bigger and smarter than them 100% of the time prior to puberty. If the eldest loses just once to the younger at something, they might never win again. That is some serious motivation.

Plus, to steal an excellent point from elsewhere, by partially reconciling with his father prior to the final fight, had Tommy lost some of the rage that fuelled him? I think that may have been the directors intention to subtley convey that

ii) why both sons are so grudging at the overtures of their father

If a parent fails you in your formative years. That doesn't go away and you have every justification if you wish to remember that failing and take it to the grave. Brendan had the best take. Be civil, be decent, but make any contact on your own terms. I dont see how Brendan can remotely be criticised for that, including if he chose to have nothing to do with the father after the fight.

If my father had repeatedly beaten my mother out of drunken idiocy (as appeared to be the suggestion here), I'd probably threaten him with a doing if he even came in the same town as me or my mother, and that's also reasonable. Putting blood (even awful relatives) before people in your life who do you right (but that you arent related to) is something I will never personally understand. The only criticism I'd have is why Tommy would have sought out his father rather than any other trainer, but that is more a criticism of the plot than of Tommy.