The question is whether the USSC ruling allows bad faith to be taken into account or confers absolute immunity.
Also a question of whether challenging the selection of his successor is within his role as president, as it doesn't involve enforcement and execution of Congress's decrees or his diplomatic duties?
If this is Trump qua private citizen / candidate, then immunity should not attach. And indeed, the president has no role in the selection of the incoming president. It's the Senate and the V-P who play the key roles.
You didn't read the case or summaries of it, did you?
Bad faith doesn't matter.
The court will decide if something is an official act.
In other words, challenging the selection (if it is Donald Trump) is immune.
There is no list of official acts, it is "what does this judge decide is an official act for this person".
(If you want the fig leaf, it is that the President's job is to ensure the elections ran properly.)
It's a power grab by the judiciary.