How many times are we going to tell you that the unit (value) will be seperated by multiple different schemes whether or not SPVP is implemented.
Also, it matters little what happens of the unit. The concern is where the value is assigned and whether that value is safely stored and congruent with the ledger.
Sound Money is a function of the ledger.
If the ledger is distributed on multiple blockchains but is kept intact and secure then the Sound Money property lives on. Lets set aside whether "the ledger" can be coherently discussed as being on multiple block chains for now. I'll continue to think of it as reconcilable multiple ledgers (via SPV), and you can think of it as only one. We both know what the other means now, I think.
A single block chain is not always "congruent" with itself. When that happens, we call it a reorganization, or at a smaller level, we get orphaned blocks.
When these reorganizations occur, what remains is the longest chain and a log entry, and life goes on. Usually when it has happened it is just a few blocks. On a faster chain it happens more frequently though.
One of the highly sought after SC is for faster transactions.
Merge mined chains are more vulnerable than those that aren't.
The
kept intact and secure part is going to need some work to manage what happens when there is a reorganization occurs that bridges outside the confirmation period.
The Blockstream whitepaper gives this period a day or two. That may be sufficient to make it mathematically impractical (maybe even if it is merge mined), or it might not. Or other folks might compete with Blockstream and offer SC with a shorter confirmation period. There is a lot of flexibility here, a SC can be most anything.
These single-ledger sound-money links can break. They may not do so anytime soon, but it is one of those risks we want to address.