Cypher, it's great that you care so much about Bitcoin, but the facts are that there is nothing you can do to stop SPV proofs. If the code were available today, any miner who wanted could start processing them and every other miner would still find their blocks valid. If this upsets you, your only recourse is to create an altcoin that is designed to be immutable. Satoshi designed Bitcoin to be able to adapt to the changing needs of the community.
Although it's true that any miner can choose to support SPV proofs (OP_SIDECHAINPROOFVERIFY), my current understanding is that the "locked coins" will only be secure if the majority of the hashpower also supports OP_SIDECHAINPROOFVERIFY.
Remember, only the soft-forked nodes can discern a valid proof from an invalid proof--the older nodes accept
all proofs as valid (that's why it's a soft fork as opposed to a hard fork). My interpretation of this is that the "locked coins" are only "locked" according to the
new protocol rules--according to the old rules the coins are free for the taking (i.e., old nodes accept all "proofs" as valid). What this means is that unless the majority of the hashpower supports the change, SPV proofs are useless because they won't be enforced by the longest proof-of-work chain.
As an example, imagine that a miner who doesn't support sidechains (and running the pre-SPV-proof code) publishes a block where "locked coins" are unlocked without a valid proof. All the other
pre-sidechain nodes will interpret this block as valid and will begin mining on top of it (let's call this Chain A). The nodes that support OP_SIDECHAINPROOFVERIFY, however, will interpret this block as invalid and will continue mining on top of the previous block (let's call this Chain B). So we get a forked blockchain
Now, if >50% of the hashpower supports SPV proofs, then eventually Chain B will become the longest proof-of-work chain. When this event occurs, the miners and nodes running the pre-sidechain code will abandon Chain A in favour of Chain B. Chain A will get orphaned and Chain B (where the sidechain coins are still locked) will become the main chain.
However, if <50% of the hashpower supports SPV proofs, then Chain B will grow at a slower rate than Chain A, and the two chains will remain forked.
TL/DR: I think network support for SPV proofs is still a political decision. It's just that instead of requiring support from a super majority of the community, it requires support from a simple majority of the hashpower (which is perhaps easier to obtain).
You are right, but SPV proofs can be combined using multi-signatures and oracles. => SPV proof and/or oracle/authority signature are valid transactions too.