I've read both and I'm not sure I know enough about your argument to challenge it directly, which is my critique of it. You're invoking these "isn't PFOF common practice" and "maybe PFOF was beneficial" questions as if they were controversies; of course, they are not. PFOF is practically universal, and it is universal because separating out non-toxic order flow is a win-win.
If I had to summon a direct rebuttal to your argument, it would be this:
I perceive you to be suggesting that the problem with Robinhood is that it uses PFOF, which means that customer outcomes are dependent on private deals Robinhood inks with companies like Citadel, which aren't transparent.
The problem with that argument is that literally everybody, including, apparently, IBKR, inks private deals with companies like Citadel, which means you can't suggest that something is distinctively wrong with Robinhood for having done so.
I perceive you to be suggesting that Robinhood would be more honest and transparent if they simply charged a commission that covered their expenses, rather than doing weird PFOF deals. Again: practically every brokerage does PFOF, because it would be dumb not to. Most brokerages charge commissions, but that doesn't make them transparent; they are still turning a private dial in their back office that trades off customer savings with brokerage profits based on PFOF deals.
Robinhood, for what it's worth, is bad, and the badness does have something to do with "no commissions". But the problem isn't shady execution; it's that Robinhood is a gambling site, not an investment site.