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.

struct CCoinsStats
{
    int nHeight;
    uint256 hashBlock;
    uint64_t nTransactions;
    uint64_t nTransactionOutputs;
    uint64_t nBogoSize;
    uint256 hashSerialized;
    uint64_t nDiskSize;
    CAmount nTotalAmount;
    CCoinsStats() : nHeight(0), nTransactions(0), nTransactionOutputs(0), nBogoSize(0), nDiskSize(0), nTotalAmount(0) {}
};
static void ApplyStats(CCoinsStats &stats, CHashWriter& ss, const uint256& hash, const std::map<uint32_t, Coin>& outputs)
{
    assert(!outputs.empty());
    ss << hash;
    ss << VARINT(outputs.begin()->second.nHeight * 2 + outputs.begin()->second.fCoinBase ? 1u : 0u);
    stats.nTransactions++;
    for (const auto output : outputs) {
        ss << VARINT(output.first + 1);
        ss << output.second.out.scriptPubKey;
        ss << VARINT(output.second.out.nValue, VarIntMode::NONNEGATIVE_SIGNED);
        stats.nTransactionOutputs++;
        stats.nTotalAmount += output.second.out.nValue;
        stats.nBogoSize += 32  + 4  + 4  + 8  +
                           2  + output.second.out.scriptPubKey.size() ;
    }
    ss << VARINT(0u);
}
static bool GetUTXOStats(CCoinsView *view, CCoinsStats &stats)