Blockchain growth projections and risks
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.