The Constitution prevents the DOJ from investigating House Democrats; 18 USC § 2383 is completely irrelevant
The answer to this question has nothing to do with the wording of 18 USC § 2383. Rewriting the statute cannot not make the DOJ investigation legal. No matter what "rebellion" or "insurrection" mean, this investigation would be illegal. The reason is simple: Allowing the DOJ to investigate and then charge members of Congress for improperly impeaching the President would directly violate the Constitution’s system of separated of powers and checks and balances. Such an investigation would implicate two of the checking and balancing provisions – the impeachment power and the speech and debate clause. The investigation would be inconsistent with the plain text of these provisions, with their broader purposes, and with how they have been applied throughout our history. These all make it clear the Constitution does not give the DOJ the power to investigate Members of Congress for impeaching the President.
18 USC § 2383, the DOJ & the “Sole Power” of Impeachment
To allow the DOJ to charge Members of Congress under 18 USC § 2383 for impeaching the President is inconsistent with the plain text of the Constitution. Art. I, § 2 gives the House "the sole power of Impeachment," and the Senate, "the sole power to try all Impeachments." Sole means one and only. In other words, the impeachment power belongs to Congress, and only to Congress.
If the DOJ could charge members for impeaching the President, Congress would no longer have sole power over impeachment. If the DOJ had this power, Members considering impeachment would wonder if they would be charged by the DOJ if they pursued impeachment. They would act accordingly. This is clearly inconsistent with the House having "sole" power of Impeachment. (Ditto for the Senate and the Judiciary. If the DOJ decided to charge a Member, the courts would then have to decide her guilt, which would influence Members' decisions.)
In other words, this scheme rewrites the Constitution to give both the Executive and the Judiciary a share of the impeachment power.
Allowing the DOJ to charge Members with a crime for impeaching the President obviously defeats the purpose of granting the "sole power" of impeachment to Congress, which is to keep the President from corrupting the impeachment process. As Madison said in Federalist 10, "No man is allowed to be judge in his own cause."
Speech and Debate, Impeachment and 18 USC § 2383
Art I, § 6, the Speech and Debate clause says, Members “shall not be questioned in any other place” “for any Speech or Debate in either House.” At first glance it might appear the DOJ investigation could be tailored to satisfy this clause. After all, impeaching the President takes more than speaking and debating. It seems the DOJ could satisfy this clause by basing its charges only on these non-speech, non-debate acts. However, this approach would be inconsistent with precedent and practice, which have given this clause a broader interpretation.
That broader interpretation is based on the purpose of the clause – to protect legislators from persecution and prosecution for doing their job. To prevent the Executive or Judiciary from interfering with legislators, courts have consistently interpeted this clause to cover all (and only) legislative activities, not just “speech and debate.” So today, even those parts of impeaching that do not involve speeches or debates would be covered by this clause.
We’ve seen this argument before!
Ironically, the President’s defenders made a related argument about whether the President could obstruct Justice by firing of James Comey as Director of the FBI, and or by threatening to fire various other DOJ officials. Their argument was: Since the Constitution gives the President the power to fire his subordinates, firing subordinates cannot count as obstructing justice. Some of the President’s defenders even claimed that he could not be impeached for exercising a constitutional power.
At the time, the President’s opponents pooh-poohed this argument, an the President’s allies embraced it. Now, however…
The Bottom Line: The Three Boxes are where it's at!
Separation of powers is often treated as boring, a mechanical exercise of classifying and sorting in the appropriate box: Legislative, Executive or Judiciary. Separation of powers is so unsexy we need a clever song with a catchy tune to make them palatable.
This question shows that separation of powers is where it's at. What seems to be a hard question becomes almost trivial once we see that is really a question about the separation of powers. The truth, as the Founders knew, is that in our constitution, separation of powers and checks and balances do most of the work. Without them, as James Madison put it, the constitution is just words on paper, a “parchment barrier,” that cannot “guard against” abuse of power, large or small.