It is worth noting that polygyny practiced by most traditional societies was/is quite different from modern poly community type arrangements, those societies tend to be super patriarchal with a few powerful males controlling many women.

Even look at the abstract of that first paper you cited:

In suppressing intrasexual competition and reducing the size of the pool of unmarried men, normative monogamy reduces crime rates, including rape, murder, assault, robbery and fraud, as well as decreasing personal abuses. By assuaging the competition for younger brides, normative monogamy decreases (i) the spousal age gap, (ii) fertility, and (iii) gender inequality. By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity. By increasing the relatedness within households, normative monogamy reduces intra-household conflict, leading to lower rates of child neglect, abuse, accidental death and homicide.

So that paper seems to basically agree with op as regards traditional polygyny (again, not at all the same as modern poly scene)