... This piece of the puzzle is easy and already well-understood. I had assumed that btc1 was going to insist on it's forking-block being over 4MB in weight, a simple and elegant solution to the wipeout risk.... They couldn't possibly be stupid enough to not do that, could they?
No idea. How can I tell SINCE THERE'S NO TESTNET TO WORK AGAINST?
I don't understand? If I have a pre-split UTXO, and I send it back to myself with their magic value in an OP_RETURN the transaction is not valid on the btc1 chain, and the resulting UTXO may be spent freely with no risk of replay, and can also be used to taint other UTXOs [At the expense of privacy]. Once the BTC transaction is confirmed, the same UTXO may be spent freely on btc1 with no risk as the UTXO is already spent on Bitcoin....
1) The OP_RETURN stuff makes the tx non standard, not invalid. In other words, nodes won't propagate it, but angry enough attackers can choose to mine them anyways.
2) With any form of opt-in tainting, the only sane path forward is sending coins back to yourself on both ends until you can verify your UTXO set divergence has confirmed on both sides of the split. Whether you do this with their convenience OP_RETURN method, or through RBF, LockTime, using coinbase transactions or whatever else you can think of, the end result is the same:
You can only demonstrably prove your coins are tainted after you have confirmations of a divergent UTXO on both chains.
Bitcoin transactions are final, replay attacks are a real threat! Therefor, there is no reasonable argument for simply trusting the opt-in protection when you can actually verify its effect.