Currently it is possible to stupidly lose mempool data by calling the rpc "savemempool" before the previous stored mempool has been read and processed to the memory mempool.

I haven't looked at the code yet, but at minimum one ugly solution would be to have a global g_is_mempool_read_finished (or something of the short) that starts false and gets sets to true once the reading is reading processing of mempool.dat is successfully finished. The rpc savemempool checks this global on the start and returns a an error while it remains false.

throw std::runtime_error(
            "preciousblock \"blockhash\"\n"
            "\nTreats a block as if it were received before others with the same work.\n"
            "\nA later preciousblock call can override the effect of an earlier one.\n"
            "\nThe effects of preciousblock are not retained across restarts.\n"
            "\nArguments:\n"
            "1. \"blockhash\"   (string, required) the hash of the block to mark as precious\n"
            "\nResult:\n"
            "\nExamples:\n"
            + HelpExampleCli("preciousblock", "\"blockhash\"")
            + HelpExampleRpc("preciousblock", "\"blockhash\"")
        );
    std::string strHash = request.params[0].get_str();
    uint256 hash(uint256S(strHash));
    CBlockIndex* pblockindex;
    {
        LOCK(cs_main);
        pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    CValidationState state;
    PreciousBlock(state, Params(), pblockindex);
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, FormatStateMessage(state));
    }
    return NullUniValue;
}
UniValue invalidateblock(const JSONRPCRequest& request)