If the specific statements attributed to Caldwell about trying to snuggle weapons into the Capitol, etc., are accurate, then he can/should be charged regardless of whether he was an Oath Keeper or ended up actually entering the Capitol. Yep. But remember, this may be a case of, "Trust us, this is what the decrypted messages say" without any way to verify that's what they actually say.
Well sure - that's why I qualified that with "if the statements are accurate." If they are not, and have been doctored, then I imagine he'll be able to get enough money that a forensic analysis would show that.
First thing to look for his an express denial from him or his lawyer.