Your example is flawed. You say that the transaction fee will be only 1/10 of a penny. What happens if Ripple grows and new XRPs are not handed out in the system? The value of XRPs will go up without bound. Especially because, once an account is funded the XRPs are "locked in." From an analysis perspective they are effectively destroyed since they can't leave the account. That means that the rate of deflation is many orders of magnitude greater than the rate of XRP destruction due to transaction fees. "Implausible in the extreme" is only true if no one controls the bulk of the XRP supply.
50 billion XRP will be given away by Opencoin. But it still doesn't matter. So long as there are enough XRP in circulation to perform transactions, XRP still won't limit the ability of people to perform transactions. Over a quadrillion drops have already been given away, and consensus can change the base transaction fee to less than a drop.
I keep hearing about how easy it is to change the transaction fee through the consensus mechanism. But no one has explained exactly what criteria a gateway will use to determine if the fee needs to be adjusted.
The primary criterion will be what they believe the transaction fee should be. The secondary criterion will be what they believe they can get a consensus on.
I believe I explained the mechanism in more detail in another thread, but here's a summary:
1) Validators add to specific validations an indicator of what they think the transaction fee should be.
2) Validators look at other validations to see whether there's a sufficient consensus for moving the transaction fee up or down.
3) Validators then add pseudo-transactions to change the fee.
4) If a pseudo-transaction gets voted into the consensus transaction set, fees change as of the next ledger.
A similar mechanism is used to make logic changes, except nodes won't themselves vote yes on a change unless they see it has a supermajority or if they are specifically configured to treat the change is urgent.
There is no specific check for spammy transactions. If it has the correct fee, it will be processed. If the network is overloaded, slow nodes will bow out of the consensus process and the transaction fee will rise to maintain stability.