Blockchain growth projections and risks What is the projected block chain growth over time and could an attack on Bitcoin be lauched that creating an enourmous number of micro-transations to fracture the currency (e.g. spamming the system with valid nuisance transactions)

I cannot believe the pruning the blockchain makes any sense (ultimate source of the bitcoins would be obscured requiring use of post-pruned bitcoins to be trusted as if these were mined at the time of the pruning)... and I cannot see using some sort of distributed storage working (trust and centralization of information concerns).

But even if pruning were possible - it seems to me that a concerted attack would still be able to overwhelm the system in a relatively short time... because transaction generation is cheap as compared to validating these spamish transactions.

Is there an overall implied governance on the system for the number of transactions that can be processed in any period? e.g. the number of systems solving transaction validation problems versus the number of transactions injected into the system... and as the volume of transactions that are needed to for validation, some (many?) systems might become overwhelmed and unable to store / process the blockchain... but even in the case of legitimate growth in utilization I think about large international retailers who process billions of transactions per day what would the breaking point be should a sudden shift to bitcoin occur? e.g. what are the on-boarding costs for the bitcoin network for new retailers?
There's a maximum block size (in bytes). So growth is limited to a linear growth and pretty predictable.

Spamming the network is very expensive because the spammer would be paying a lot of fees.

The current blockchain size is not that bad: it fits on a simple SD card if you want. And storing it on a harddisk costs cents.

Pruning can be done in such a way that there is very (very!) little room for manipulation. Everything is cryptographically provable. And there will always be full nodes around that do keep the full history and can check everything. Others would probably be able to query those nodes to provably check the complete path of particular coins. If you can do that on a even a random subset of coins, you can be pretty darn sure there was no manipulation.

Your last paragraph indicates that you don't fully understand the block validation and mining process (like the maximum block size and only 1 block every ten minutes on average). Maybe you need to look into that some more.

Regarding scaling the network up: there's still quite a bit of room in the current block size configuration, but the developers can fairly easily increase that maximum.