I think it would be helpful (in general) for me to paste here my updated understanding of how these "full-security new lite-nodes" would be booted up:
For a new node to boot up from scratch and begin securing the network, approximately the following needs to happen:
1. Download entire BTC blockchain headers.
2. Download the entire UTXO meta chain.
3. Begin merged mining on UTXO and BTC blockchains. begin mining *only* when we've built up the complete UTXO tree.
4. For each new transaction the "almost-full node" receives, query other nodes for the block in which the payer's bitcoins were previously located, along with the hashes to verify the merkle root in the BTC blockchain. Verify the root can be found. Edit: This part is just regular SPV!
5. Query other nodes for the transactions and merkle root hashes that result in the merkle root hash of the most recent block in the UTXO blockchain. Verify that previous txn the coins belong to exists in the current data comprising the most recent merkle root of the UTXO blockchain.
6. If the transaction passes the above verifications, store all received data so far in order to build the Merkle hash tree that represents all the UTXOs.
To build the complete UTXO tree, ask for any missing children of our current UTXO merkle tree and verify them according to the above.