Could a google or facebook or paypal get a competing digital currency going, then use their computing power to wipe out BC value with 51% computing power?
The so called 51% attack (which is actually the >50% attack) doesn't require a competing currency (just the hardware hashing power) - what such an attack potentially can do is either "undo" some tx's or refuse any new tx's (the latter being perhaps of more concern). In either case if such an attack were to be carried out it certainly would be an issue for Bitcoin (please also note the that
Bitcoin is one word not two) although not necessarily fatal (ideas such as "proof of stake" are already being trialled in other alts). Also note that the cost (in terms of hardware) to carry out such an attack would be rather high.
Could the open source aspect of BC at a later date choose to increase BC supply?
It is a trivial change in terms of source code but without 100% of current users agreeing to migrate to the same new software it would result in a "hard fork" (which would turn Bitcoin into effectively 2 currencies at the point that the new software was used).
When USPTO goes first to file, could someone patent the source code? Under the First to Invent system, public disclosure sets a 12 month time clock to file before forfeiting proprietary rights and rendering technology public domain. Also, my understanding of existing patent law, is that anyone that contributes materially to an invention has equal rights to claim ownership of proprietary rights, meaning that anyone that contributed to the source code could file a patent. I don't know how the new First to File will change public disclosure laws.
Although IANAL the software was initially released in 2009 (and contains "Copyright (c) 2010 Satoshi Nakamoto" comments in the source code) so unless a patent was filed in 2010 I think it would not be possible to do so now.
Cheers,
Ian.