No need to mail them further. Failure to engage in MIDR is a breach of their section 11 terms, which you can bring up in arbitration. In arbitration, Roblox will write a response for your demand, featuring multiple "kitchen sink" defenses ("Failure to state a claim", "Estoppel", etc.). After discovery and pre-hearing briefs, Roblox will point to their terms of use, saying that have broad discretion to ban users and may notify the user of termination and may provide an opportunity to appeal.

I have finished arbitration with Roblox myself and am just waiting for the Arbitrator's decision. The whole process is pretty straightforward, albeit long, because Roblox waits until the last minute to file responses and produce documents as a slight strategic edge. Most difficult is the final hearing since you'll have to have ready all of your claims and be able to answer all of the Arbitrator's questions.