In the bookworm raspberry pi repository:

Regarding Question 1:

Question 1: Why is there available both tk-dev and tk8.6-dev? Are both v. 8.6?

Both tk-dev and tk8.6-dev are version 8.6. It seems that tk8.6-dev is a later version, unless I'm reading that wrong?

tk-dev/oldstable 8.6.11+1 arm64
  Toolkit for Tcl and X11 (default version) - development files

tk8.6-dev/oldstable 8.6.11-2 arm64
  Tk toolkit for Tcl and X11 v8.6 - development files

And this from

https://www.tcl.tk/software/tcltk/8.6.html

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Latest Release: Tcl/Tk 8.6.15 (Sep 13, 2024)

First released in 2012, Tcl/Tk 8.6.* is the current supported series of releases.

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Therefore, I will not use the tk-dev, but instead will get the tk8.6-dev and tcl8.6-dev from the default pi bookworm repository, before building any future python 3.12.5 or greater. In case I find it helpful to ask the raspberry pi folks for further support, I am sticking with their repository versions of these libraries rather than getting the absolute latest from www.tcl.tk. I may change my position on this after some testing and further research.

Regarding Question 2:

Question 2: Can you detail anything I might be missing in this plan, or anything that I do not need to install before the make altinstall of python 3.13, for ensuring full tkinter 8.6 support in python 3.13?

This question remains unanswered, and I add to it -- I intend to access and play audio files using the final tkinter project. Does anyone have helpful remarks on needed libraries, before the python 3.12.x or python 3.13.x build, that might be needed to ensure full robust audio support in a tkinter project?

Thank you for your thoughts on these matters.