Let's ignore the crypto-currency part of Bitcoin, the 51% attack you're talking about is a form of computer hacking, and there are laws to protect against this.
My opinion is that any law that applies to "hacker 'penetration testing'" applies to hacking bitcoin in addition to whatever else the law can throw against such an attacker.
Don't do it! Don't do it! If you are in the US, the law is very broad.
You don't want to even tiptoe up to the line.
The relevant law is the Computer Fraud and Abuse Act (18 U.S.C. 1030).
In a nutshell (and simplifying slightly), under the CFAA, it is a
federal crime to "intentionally access a computer without
authorization or exceed authorized access". This language is very
broad, and I imagine an ambitious prosecutor could try to use it to go
after everything on your list except #1 (view source).
Orin Kerr, one of the leading legal scholars in this area, calls the
statue "vague" and "extraordinarily broad", and has said that
"no one actually knows what it prohibits".
And, as @Robert David Graham explains, there have been cases where
folks were prosecuted, threatened with prosecution, or sued for doing
as little as typing a single-quote into a textbox, adding a ../ to a
URL, or signing up to Facebook under a pseudonym. It's pretty wild
that this alone constitutes a federal offense, even if there is no
malicious intent. But that's the legal environment we live in.
I'd say, don't take chances. Get written authorization from the
company whose websites you want to test.
Although that above link applies to a specific company, I'm sure the distributed nature of Bitcoin could land a hacker in hot water in several countries, not to mention the attacker will also be wanted by the entire community of Bitcoin users (both good people and unsavory users).
If you want to test, be open and transparent about your actions (tell people on the forums) and only do your tests on the test network. (yes there is a dedicated bitcoin network for testing that makes it pretty easy to apply a 51% attack on)