[Iraq is a monument to English colonial stupidity. This region has been a battleground between Shia (Persia) and Sunni (Arabs) for 1400 yrs. Creating a geographical country only made it worse.]
The idea that the European powers are responsible for the strife is a monument to magical thinking combined with Soviet fellow traveler narrative that looked for any reason to blame the capitalist West. As you noted, local ethnic and religious disputes pre-date their independence by thousands of years. The era before European governance intermingled all these people. Each group wants its traditional lands plus where they’re living today. When you add all the territorial claims up, the demands are several times the land available. Partition means instant war, the moment the occupying power leaves. So they drew lines for defensibility and a rough balance of power and hoped for the best.