We were essentially discussing Sabunir's question about what prevents someone from messing with the program's source code to adjust block-generating difficulty to be very easy, then make a network on his own and create a, say, 50,000-block proof-of-work within seconds then finally propagate it across the real network to steal "votes" towards his new fake blocks as technically, his proof would be "the longest". So is there a way to verify how much work was actually put into a given PoW (for eg. how many zero's are at the beginning of each hash or something)?

Since we're at it, what's the approximate time for proof-of-work propagation across a network of about 100,000 machines? Is there a way to optimize connections so that broadcasting is done via a pyramid-form to minimize the needed time? For example, the block creator sends it to 10 nodes, then those 10 send it to a 100 provided that none of those 100 were among the original 11, then those 100 tell a 1000 provided that none of those 1000 were among the original 111, etc to save time.