Wow, there is a lot of misinformation in this thread.
First, this was not preventable by a $500,000 valve. I suspect that this comes from the fact that Brazil and Norway require a remote actuator for the blow out preventer and the US does not. These have been widely quoted as costing $500,000 and one was not in place for this well.
The remote actuator doesn't stop the flow of gas or oil. It is used to trigger the blow out preventer (BOP). It is the BOP that actually shuts off the well. The BOP is normally triggered automatically whenever the pressure or flow rate exceeds a preset limit. If that fails, there is a remote cabled switch on the drilling rig. If that fails, there is a manual switch on the BOP itself. The advantage to having a remote actuator is that it can be triggered by someone not on the drilling rig if there is an accident that prevents the switch in the rig from being used. Without one, when a blowout like this occurs and the BOP doesn't get triggered automatically or by someone on the rig, you have to wait until a remote vessel can dive down to the BOP and trigger it.
In this particular incident, having the $500,000 remote actuator would not have helped. The BOP itself was faulty. Triggering it wasn't the problem. It didn't work. At this point, we don't know why. There are indications that some portion of the BOP may have been leaking hydraulic fluid and been faulty. There are also reports that junks of rubber from the BOP came up with the drilling mud, indicating that it had broken.
This will unquestionably be damaging to the environment. How badly or for how long is unclear. A similar incident occurred in the Gulf of Mexico in 1979 off of Mexico. The oil slick hit Mexico and Texas. It was nasty. I remember finding tar balls years afterward (although some of those may have been caused by natural seepage or other sources). The environment survived and there are few traces remaining today. That was about 30 years.
Should Mexico be compensated? It seems unlikely. The current flow patterns appear to be taking the spill north and east, so Mexico will probably not be affected except for the impact on migratory birds and marine life. Even if Mexico is affected, it is worth remembering that they refused to pay any claims related to the Ixtoc spill, claiming sovereign immunity.
The $75 million cap applies to damages to private parties. BP is still on the hook for all of the cleanup and remediation costs. The cap comes from the Oil Pollution Act of 1990. The act created a fund that is to be used to pay for damages. Every oil company has been paying into the fund since the act passed. It is that money that should cover the cost to fisherman, beach hotels, etc. Congress has talked about changing the law and retroactively applying it. Personally, I think that the liability limits are too low, but I abhor the idea of changing laws retroactively. Doing so corrupts the whole concept of being a nation ruled by laws.