Firstly, about the alleged discrimination he faced because of being a SutaPutra and not being admitted into Drona's Gurukula+
A Sarathi readies the chariot. Makes decisions on the horses to yoke, number of bows and arrows to keep, how many kinds of arrows to keep and how many of each type. A wrong decision+
FYKI, Duryodhana and Bhima too were not there in the list. AFAIK Arjuna, Yudhishtira and Ashvatthama were there+
The effect of the curse can be debated as Karna, despite loss of memory, manages to invoke Brahmastra twice and Bhargavastra too.
It is Arjuna's arrow powered by the merit of his humility towards elders that kills Karna+
Karna was a better archer, probably w.r.t Yudhishtira and twins. His victory over Bhima was a near impossible one and Bhima earlier and later defeats him in the war.
One fact. When he defeats Bhima+
Arjuna, Satyaki, Bhima and Abhimanyu.
And there were countless instances where he could have been killed but was saved by Ashvatthama+
IMHO, it is not Duryodhana's friendship that threw him down (many sided with Duryodhana), but his inability to overcome narrow minded rivalry with Arjuna.
I have closely observed some Karna glorifiers. A common trait in them is that+
It is a cycle that forced them into mediocre whiners while they had the potential to excel if only they kept at it.
Judging them without nuance and learning is just gossip. Will continue later