Well, you could have said exactly the same things about Bitcoin back then when it was just a paper: how nodes in the Bitcoin network are forced to select the longest chain? No one force them. Preferring the longest chain is just a convention, yet if everybody else act according to this convention, you better follow them or else your mining becomes meaningless. The same is true for GHOST.

You asked how nodes will be informed of all tree blocks, and is done simply by propagating valid blocks like today. As there are more blocks to propagate indeed there is a higher chance for someone to miss a block. I think that the standard protocol solves that continuously comparing the list of recent block hashes with the node's neighbors, yet this can be further improved by including previous off-chain blocks' headers within each block (those blocks are off-chain with respect to the chain forms by the block that contains their headers). In my suggested implementation blocks are actually incentive to inform about competitive chains!
 
No one doubts that the original Bitcoin creation is genius, yet even Gavin Andresen is saying that it will not last forever without modifications. So we better be open-minded to look for the next genius creation  

Lear