Parenthetical sentence (double commas) followed by definition in parenthesis - where to put commas?

Because your legal example is a sentence fragment, it's hard to be sure. More clear is the example about your house. To me,

"My house, which is adjacent to another house, (the 'House') is nice" is wrong because, as you say, "(the 'House')" goes with "another house".

I would either write

"My house, which is adjacent to another house (the 'House'), is nice"

as in your other example, or I would have commas on both sides to match the natural pauses you would include when speaking the sentence aloud:

"My house, which is adjacent to another house, (the 'House'), is nice"

Part of the problem here, I think, is that you are using an extremely abbreviated form of the parenthetical idea intended: "(the Act)" is short for "(hereinafter referred to as 'the Act')." Neither of the two versions that you offer as ways of dealing with the specific situation you ask about—

Version 1: "according to the U.S. Securities Act of 1933, as amended (the "Act"), the Securities...."

Version 2: "according to the U.S. Securities Act of 1933, as amended, (the "Act") the Securities...."

—does a very good job of representing the intended meaning, it seems to me.

In the first place, the point of the wording is to set up "the Act" as a short way of saying "the U.S. Securities Act of 1933, as amended"—and yet both of the versions you offer leave the outside the quotation marks, as if you expected to write things like "When Act was signed into law..." and "The key provision of Act on this question is..." This problem is easily corrected by including the word the inside the quotation marks: ("the Act").

In the second place, I think it is a bad idea to allow commas to interrupt the full phrase that you want the short form to represent. In Version 1, you let a comma stand between "the U.S. Securities Act of 1933" and "as amended," inviting confusion as to whether "('the Act')" refers to the eight preceding words or only to the two following the comma. Version 2 is even worse, allowing two commas to clutter the expression that is to be shortened.

In place of either of those versions, I would recommend this one:

Version 3: "according to the U.S. Securities Act of 1933 as amended ('the Act'), the Securities...."

Whether the author would normally set off "as amended" with commas is beside the point. In the particular case where a writer is introducing a parenthetical short form of a longer string of words, including intervening punctuation (here, commas) is an encumbrance to clarity and coherence— and what is the point of punctuation if not to make the author's meaning easier to grasp?

In terms of editing translations....

The full citation should read:

"under the U.S. Securities Act of 1933, as amended (the Securities Act)," [rest of sentence]. Alternative: pursuant to

At first glance the above style seems odd; yet it is used by major law firms and in SEC filings.

"According to" is not a good choice here for SEC filings. under or pursuant is better. (according to sounds like someone is giving a version of a narrative so it's best avoided here.)

Harvard Law:

Information delivery requirement—For resales of securities of an issuer that is not an SEC reporting company or exempt from reporting pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), certain information must be delivered to a prospective purchaser. The required information includes, among other items, “reasonably current” financial information [3] prepared in accordance with GAAP or IFRS but which need not be audited or reviewed.

From the following ladeedah law firm: Posted by Nicholas Grabar, Cleary Gottlieb Steen & Hamilton LLP, on Saturday, January 16, 2016

FAST Act amendments

Rule 802 under the U.S. Securities Act of 1933, as amended (the “Securities Act”), provides an exemption from the registration requirements of the Securities Act for certain cross-border exchange offers and business combinations by foreign private issuers involving the issuance of securities. (Law firm: David Polk)

Some law firms etc. use: as amended (the Securities Act), [rest of sentence] and others use: as amended (the "Securities Act"), [rest of sentence]

That said, in this particular case, law firm and official usage is what counts. The usage is well established.

Finally, one also sees: the U.S. Securities Act of 1933, as amended, [etc.].

The OP did not provide a full sentence so the discussion is not moot.