You put this in the FOREX forum. Are you only interested in the "Best Broker for U.S. Retail Traders" for FOREX? SEC brokers dealers can't offer retail FOREX.
Yes. So forex.com and oanda are operating illegally in USA? What are you saying exactly? And they have little to do with SEC, they're regulated by NFA and CFTC.
You can open an account in the USA with IG https://optimusfutures.com/tradeblog/archives/trade-forex-with-ig At the bottom of the article you can find a demo and live account links
IB requires 10M to trade forex. Get a offshare broker like Coinexx, FXbrew,with 500:1 leverage and 0.02pip spread(on euro). They are not regulated but many traders traded them for years with no problem. If you are limited to regulated US forex brokers,you are going to tolerate low leverage and higher trading cost.
Did You personally trade them to see how they handle quotes, requotes, SWAP rates, slippage and price spike and above all Withdrawl policies? When you say "Many Traders" do you know their personal experience? In both the USA and many countries in Europe (Under ESMA) the regulators have lowered leverage to avoid situations that we had with the Swiss Franc where many had negative balances that many experienced.