According to their site, they'll have an official API in the "near future", and hopefully one that allows trades to be placed. Granted, they've said that for almost two years now. I fully expect to die of old age before it's available.
Robinhood isn't a darkpool - they sell their "dumb" retail flow to the highest bidder wholesale to the likes of Citadel, Virtus, UBS, etc.. etc... Usually they just pick 1. This type of flow is called "PFOF" (pay for order flow). These PFOF players also provide marginal price improvement over the NBBO... marginal - because they always make more money than they pay out - and take marginal risk.
I don't know.. but it's a good question. Matching internally first creates huge overhead, compliance considerations, etc... Unless you already have the right infrastructure in place. For example, you need to report the trades. My guess is they wouldn't do this...