Borrower defaulted and there is strong evidence of promissory fraud. How can the annuity plan account and 401k account best be reached?
Generally speaking, a security agreement purporting to use a 401(k) account as collateral is void as a matter of law unless the loan is made by the institution that maintains the 401(k) account. This is also true of many annuity accounts which have special retirement account character (e.g. a TIAA-CREF annuity as part of a teacher's retirement plan).
A non-retirement private annuity could normally be foreclosed upon in some manner as security, by forwarding the note, evidence of default and the security agreement to the private annuity holder, assuming that the security interest in the private annuity was properly perfected under the Uniform Commercial Code Article 9. If it was not properly perfected, it still might be possible to access this assets in a court action to enforce the promissory note and security agreement.
Default would be sufficient to get a money judgment under the promissory note, and the mere fact that a 401(k) was pledged as collateral when it was not legal to do so would not show promissory fraud, without a showing that the borrower knew this and intended this effect. Otherwise, it could simply be a mutual mistake as to the legality of that security agreement. A court might very well find not fraud but negligence on the part of the creditor for not taking the proper steps to obtain collateral status for the collateral under the Uniform Commercial Code.
Even if there was promissory fraud, this would only prevent discharge of the debt in bankruptcy if the creditor made the proper showing in a contested adversary action timely filed in a bankruptcy and would generally not permit access to the 401(k) funds. There are very few exceptions to the immunity of 401(k) funds from creditors claims and nothing in the OP suggests that the facts necessarily to benefit from any exception would be present in this case.