This package does nothing with C-n. Smooth scrolling at high speed can certainly take some CPU, nothing new there.

Did you either succeed with debugpy.listen()? Is that suitable to call interactively then quit (like iPDB)?

M-x find-library python, followed by C-s outline- shows what's happening: python-base-mode sets the local outline variables locally, in its mode body. Since mode hooks always run after the body of their mode definitions, you could add a bit of code to the python-base-mode-hook (or a parent of it, like prog-mode-hook) setting your desired outline config there (i.e. overwriting what python mode did).

I do that using outli simply (in a use-package stanza):

 :hook ((prog-mode text-mode) . outli-mode))

BTW, here's a handy guide (from this epic answer) to what runs in which order upon invoking a derived major mode like python-ts-mode:

So when we call (child-mode), the full sequence is:

(run-hooks 'change-major-mode-hook) ;; actually the first thing done by
(kill-all-local-variables)          ;; <-- this function
,@grandparent-body
,@parent-body
,@child-body
(run-hooks 'change-major-mode-after-body-hook)
(run-hooks 'grandparent-mode-hook)
(run-hooks 'parent-mode-hook)
(run-hooks 'child-mode-hook)
(run-hooks 'after-change-major-mode-hook)

Not sure your definition of easy, but this kind of setup (and much more) is certainly possible: https://www.masteringemacs.org/article/demystifying-emacs-window-manager

Start with the Elisp intro; it’s great. M-x shortdoc buffer gives a nice overview of buffer commands. I love consult-info for general searching of the Elisp (and other) info files. But M-x apropos-function is builtin and useful too.

No need for a package, this is a great chance to learn some elisp. To get you started:

(let ((cur (current-indentation)))
  (push-mark nil t t)
  (while (and (not (eobp)) (= (current-indentation) cur))
    (forward-line 1)))

Seems like in-process debuggers like iPDB are far more practical for interactive debugging of long running shells.

How would you attach a DAP python debugger to a running instance of (i)Python? Is there some import debugpy; debugpy.start() command or similar?

Set a window-configuration-change-hook temporarily, see if the (selected-frame) matches. If so do the split, and remove the hook.

When I hear of a package that may be interesting, I immediately check its repo page to see how many issues and PRs are still open. I look to see whether they have garnered any responses, especially if the submissions are of high quality. Years of issues building up isn't a good sign. This isn't 100% reliable, as different skilled developers approach issues and PRs quite differently, but it gives you some information. And there are outliers, like multiple-cursors, whose developer is very skilled and motivated, but whose popularity overwhelmed his resources.

For simple package, "no updates" for 5 years is usually fine. But before investing energy in a larger new package, I want to know whether it will still be working well in the next 5 years.

There are a variety of "self-quoting" constructs in elisp (including all :keywords!).

(eq (quote nil) nil) ; ==> t  
(eq (quote t) t) ; ==> t  
(eq (quote :some-keyword) :some-keyword) ; ==> t

Outline mode works well for this. I use my own small outli package to set this up automatically with nice formatting and “speed key” access at the beginning of headlines. Tab to fold.