Non BU blocks are still 100% compatible with BU so if BU doesn't have more than 50% of hash power, the fork will fail untless BU miners intentionally filter the non BU nodes. The only way for a fork to be successful is if the BU miners mine a block > 1 MB and have 51%+ of the hash power. Personally I'll wait until 18 blocks longer before I really consider the fork successful.
Yes, you are right (see the edit of my previous post). This "backward compatibility" of BU with respect to the standard protocol makes that the standard protocol is a soft fork of BU. Then they expose themselves AT ANY MOMENT to be orphaned, even 6 months later, if ever the BU fork falls beneat 51% hash rate. Then you will get the bitcoin chain orphaned 6 months behind, because from the first >1MB onwards, the soft fork is not compatible any more, and this will be the last valid block on the chain. Major clusterfuck !
The only safe way of making the BU hard fork, is by making it incompatible with the old protocol. If they don't do this, and they "pull the trigger", this will be a major disaster on the chain.