The signature is not broadcast to all plasma watchers, because that would allow any sender to hold up the system by not broadcasting their signature. Rather, if you receive a UTXO, then you need to show the confirm sig for the UTXO at the time that you spend the UTXO. Slightly different mechanism, but same effect.
Why not wait for somebody else to do this challenge and safe on transaction fees (Tragedy of the commons)?
If we want to, we can require participants to submit an additional deposit upon joining the system, and give this deposit as a reward to those who challenge.
You should even check all blocks for validity
Exactly correct. And if you notice even one invalid block get accepted, you exit immediately (or at least within 7 days).
If all users try exiting at the same time, it can lead to a huge spike on the parent chain, and increase gas costs, so they will have to pay $10 to exit instead of regular $1 - it can be something very much similar to a short squeeze on capital markets …
This is indeed the fundamental flaw in all channel systems, raiden and lightning included, and is the reason why the scalability of this system can’t go too far above the scalability of the main chain. 2-3 orders of magnitude probably but not that much more.
There is a potential race condition between:
You’re right. One simple way of fixing this is to require a minimum waiting period between consecutive submitted blocks, so if you want your deposit would be safe you would submit yours right after the plasma chain submitted a new block, so that it would with quite high probability get included on time.