I like the car analogies they've worked well for me. So "spam " just being a term for denial of service or an overload of inappropriately priced transaction data, leaves me thinking it isn't quite wishful thinking making room for it.

Reflecting on the  issue has lead real smart people to find solution that I think are nonsense (picking on Sidechains here) in my experience sometimes jumping in the deep end and being overwhelmed leaving one focused on solving the issue as they happen necessity is the mother of invention and I think allowing it is a great way to do RnD, something I feel is partly in keeping with the spirit of Bitcoin.

One suggestion. Is it worth mentioning IBLT because by the time 16MB blocks are being mined they will likely be taking up just 500KB of bandwidth overhead to propagate. Obviously the real-time unconfirmed transaction overhead does remain. and is large long-term.

Pegging two different, however slightly, money types together is a problem in theory and practice. As long as they are different, they will have different value. If the value is smaller, they will be converted to bitcoins. If the value is larger, they will be converted to sidecoins.

When a fiat system goes bust, a new one is created, and it is quite common to peg the value to for instance the dollar. This is to try to give people a reason to trust the new money. The reason to have local money at all, is to get the seigniorage, which otherwise would be wasted to another government. To peg the value, you need an institution to be the guarantor, that means a kind of bank with reserves in the other money type. In practice, exact pegging is not possible, because there will be leakage of value to speculants and money exchange services. Therefore there will be a band, where the value will be pegged to somewhere between the upper and lower limits. Even better, the exact limits are secret, to avoid speculation near the edge. This can go on for a while, until they have created too much (sometimes too little), the peg breaks and is set to another value.

If there is a mathematical peg defined by protocol and secured by the blockchain, the difference in value will have to escape somehow, and that is either the coins disappear if they are worth less than bitcoin, otherwise all bitcoins will be converted to sidecoins. It is not rocket science.

That's false. IBLT almost halves the bandwidth requirement. Currently transactions are sent twice: once when broadcast and once as part of a block when mined. IBLT reduces the bandwidth requirement of the latter to a constant.