Merely reverse engineering for interoperability is not a violation of any type of IP in the US.

1. Reverse engineering for interoperability gives you a defense for copyright and trade secret claims in the US. It provides zero protection from patent claims. "reverse engineering a patent" doesn't even make any sense. Patents are public, there's nothing to reverse engineer. You could "reverse engineer a specfiic implementation of a patent", but the only thing you'd be accomplishing is just avoiding infringing on the parts of the implementation that are subject to copyright or trade secret protections. You'd still be violating the patent.

2. I'm not saying their arguments are valid. It's a demand letter with multiple claims. There's many reasons it might be an invalid argument but listing more than one grievance and/or not explaining them in detail are not some of those reasons.