Of concern to me is that Presidential communications to aids/advisors can’t be used as evidence. ⬅️ I don’t see why this was done at all. Only recently we found a note by Halderman(?) that indicated Nixon had tt him about contacting N Vietnam about not doing a deal with LBJ. So what gives here? Can anyone explain?

Sure.

This is direct application of the law that currently applies to other immune acts, such as the immunity granted to Congresspersons for their speeches and votes. When a Congressbeing votes, that action is entirely immune from everything - it can't subject them to civil or criminal liability. The Constitution has allotted to the Congressperson the sole decision on how to cast their votes, and it cannot be controlled or directed or dictated by anyone else, through any means.

Because immunity is intended to give them the complete control over their votes - not the judiciary and not the executive - they are also protected from any efforts to indirectly use their vote against them (outside of normal political processes, natch). So it precludes judicial inquiry into the motives, reasoning, or basis for the Congressfolk's choice to cast their vote. That's discussed in lots of SCOTUS cases, such as US v. Brewster at the below link:

https://supreme.justia.com/cases/federal/us/408/50...

At its heart, the concept of immunity rests on the idea that the Constitution has established a final framework on who decides certain questions - and that this allocation of authority cannot be usurped by either of the other two branches. So the casting of a vote or the giving of a speech in the Congress is "protect[ed] against possible prosecution by an unfriendly executive and conviction by a hostile judiciary" by keeping those other two branches from questioning the motives or justification for such votes. Neither the Executive nor the Judiciary get to subject a Congressbeing's reason or motive for casting a vote to any test to assess whether it was the "wrong" reason or motive.

The majority applies the same thing to decisions that are wholly reserved to the Executive. For example, the Executive decides what cases are prioritized for prosecution. That's part of the executive power that the President and the President alone ultimately wields. Those aren't choices that Congress gets to make, or the Judiciary - they are put solely on the President's plate, just like the decision to cast (or not cast) a vote is solely on a Congresscritter's plate. So, like the decision on how to cast a vote, the decision-making behind those choices - the President's motives and reasoning - is off-limits for the purposes of establishing a sanction or punishment.