Citadel has no short positions to speak of in GameStop. They bailed out Melvin, which means they made money, not Melvin's positions per se. Also, by the time that happened Melvin had already closed out its short [1].
The conspiracy hypothesis might have legs if Citadel's asset management and market making arms colluded. Absent evidence, however, it's a suspicion at best.
Robinhood is between a rock and a hard place of its own making. Market makers stopped making markets in an increasingly-volatile stock. Robinhood, having relied on market makers for execution, likely has sub-par exchange connections, so their execution quality started degrading.
They're also staring down almost-guaranteed lawsuits regardless of what they do. If they badly execute, lawsuit. When the bubble pops, lawsuit for having been induced to trade through their gameified UI. If they block, lawsuit for interfering with trading.
[1] https://www.cnbc.com/2021/01/27/hedge-fund-targeted-by-reddi...