Possibility of a DDoS attack Theoretically. This is not a mechanism of any kind of attack, but It's about the "lost" coins. May be. It is just for an educational experiment.

What if someone has the ability to send transactions to the each node's memory pool. He have also the ability to send TCP packets from some list of the internet protocol addresses (65533 IPs against 9686 nodes, for example).

He send  transactions, but R and S in it's signature are not formed by the such way as they can be formed by the real owner of the private key. Each node needs to get unsigned TX hash and then check it's Sig for to confirm the output's amount spending. It will take some time.

Is it a theoretically possible DDoS attack to [any]coin network? Each node need to verify signature and it takes some time. Provided that attacker has the full UTXO set locally on his [XXXX] machine IPs.

Or IPv6 folds this "problem"? Or what is really means "bitcoin full-node"?
Each full node maintains a score for the peers it has connected to called banscore. Any 'mischief' on the part of the peer result in an increase of the banscore. When banscore reaches 100, the peer is in most cases disconnected.

When a node receives a transaction from the peer, it will verify the transaction and signature check forms a part of this verification process. If a malformed transactions gets continuously sent from a peer, the node will simply disconnect itself.