First 51% and 1tx block have nothing to do with each other. 51% attack can happen even if botnet includes all txs.
It does, depending on how the botnet is handling tx. If the botnet is running off a few servers providing the blockchain, then that botnet would increase the amount of computing power needed to hit 51%. As it stands, since the botnet is blind, it can be incorporated by an intelligent attacker to make up 51%. At the current botnet size, it gives about 15% leverage either way, although this may increase or decrease in the future.
Unless mystery is an idiot he is going to start saying ... "I would like 6% more revenue lets see how I can get a cut". If he doesn't then maybe 8%, 10%, 15% eventually drives him to the tipping point.
Someday fees will need to support 100% of the network. We can start the ball rolling at > 0.03%.
I haven't seen a single proposal that would either work or wouldn't have significant side effects and open the network up to "gaming" and malicious attacks. Protocol changes should be the last choice. Miners requiring higher fees is a simple strategy to try first.
Well, in 2013, when that becomes relevant, then we can talk about what a reasonable fee is. 1 bitcent isn't bad, but it can't be predicted how the value of BTC will respond to the drop in subsidy, or what kind of losses that will mean to the miners (or not). If the value of 1 BTC goes to $10, then an even smaller fee would suffice, since technically the miners wouldn't be losing anything.
Let's just say mystery saves, say, 25% of costs by
not running the blockchain, and the value of BTC doubles, then he will still have no incentive to play nice, and may even decide to take a larger chunk. The only way that would
not be the case is if bitcoin's value appreciated more slowly than the subsidy decrease, or if fees became fairly exorbitant.
The main problem is that, for the time being, the BTC community is fairly vulnerable, and it is precisely during this time when that 15% of total processing on the network would be the biggest liability to BTC's survival. If someone with the will and the resources figures that out, we could have a big problem. Even if not, that's still 15% of all BTC produced going towards someone who isn't investing in the actual function of the network. Ideally, the means should be provided so that tx's can be mined properly without the cost of a full blockchain, then mystery would be adding 17.5% or so to the total processing power of the network rather than wasting 15% running in place.
Also, I don't see how gmaxwell's proposal, with my revisions, would open up the BTC network to "gaming". Every block must be cryptographically valid as well as providing proof that it's running on correct blockchain information. It's pretty hard to make an attack within those constraints.