ernst.w

It does basically the same as Ctrl+Shift+Click does in InDesign to make a local override of a master page item. When you detach edit, I think it is basically equivalent to overriding all master page items on a specific page (you can also revert to master page items similarly as in InDesign).
Certainly, so something worth considering, if this is mainly for archival purposes...
Yes, true. Partially because of this discussion, I just made a decision to go for an Intel NUC 12 Pro to make it our main "server" to be accessed mostly with laptops (to back up and ultimately replace a full-size tower and big-screen workstation from 2014 that is now coming old). For us, too, it has become time to downsize but it is nice that one can get so much packed into such small size (and without spending too much). I hope it can run our non-subscription based work software as long as we need it but I have multiple plan Bs should that fail! Having all essential in such a compact form with fully replaceable parts inside, and then just backing up the bits on the cloud and local wireless disk gives some peace of mind. And a chance to finally get rid of a row of old computers with old OSs and legacy software just kept as a backup should something go wrong... 

I think you can edit a master-based content only as per page-basis in Affinity Publisher (you can do this by right clicking the master layer in the Layers panel and choosing Edit Detached, and then accept changes by choosing Finish). Affinity Publisher does not have global layers so they behave differently from ones used in InDesign.
Optical kerning is not supported, only metric-based. The text would be wrapped much differently anyway and I doubt there is anything that can be done to fix this (other than using manual line breaks).

Just out of curiosity, when do you expect this to happen? As far as I know Adobe still resets installation count of CS6 apps on direct chat connection -- I did this a year ago when I installed the whole master package on Windows 11 Pro, and got the installation count zeroed, so I expect to be able to install yet on a few more Windows computers in the future. As long as they do not stop the activation servers (which I think they have done with versions earlier than CS6). I have no idea whether installed and activated CS6 apps can be transferred on a new computer, but if you manage to get your CS6 suite installed and activated on a replaceable SSD of a new Windows 11 Pro computer, I guess just restoring the disc image would keep your CS6 package alive for yet another 10 or 15 years...  

Hello @ernst.w,
I hope you realise that Publisher is not going to open inDesign files with 100% accuracy.
For smaller documents this may work but for long documents you are going to run into a lot of problems.
It might be much quicker just to start from scratch in Publisher and import files from in Design as you go along.
There is no 1 to 1 translation between these 2 programs.

As I understand, IDML is a documented standard (not sure if there are any licensing issues, tho). The official documentation is downloadable at adobe: https://wwwimages.adobe.com/content/dam/acom/en/devnet/indesign/sdk/cs6/idml/idml-specification.pdf
It is pretty straight forward XML, and thus easy to read plain-text. I'm not a programmer, but I understand the basic structure and could tweak a doc in my texteditor... 
 
So the main difficulty for 3rd party developers like Affinity lies in the function set of their programs. Everytime adobe implements a new feature which affinity does not yet have itself, they have to "interpret" it with their own tools. Which is difficult and error-prone. 
Affinity already does a great job for my CS5 files, although sometimes lazy bugs occur, like converting mm to pixels (corner radius or shape). Cant speak about shiny new features of CC (2017...2019)
 
I think the main difference of IDML to INDD is easy to see in the Finder: The IDML is way smaller because it has no embedded thumbnails of the placed images and maybe other metadata to quickly relocate images and fonts. Unlike IDML, INDD is a binary format not plain text.
 
fun fact: It's a also good method to clean your documents if you notice strange bugs in decade-old and often re-used documents. (just export as IDML and reopen in ID)

I think you need particularly the import from InDesign CS6 format. Many users do survive with this last perpetual version hoping and praying to be able to finally switch

Please see Below Ernst
 
Ernst I notice that you did not respond to jmwelborn's questions.  Just in case you were too tired and might have missed it, here was his comment
 

The filler text had me confused at first as it could not be formatted or changed, but there is a very useful option in Edit - Preferences - General - Insert filler text as text. Then filler text can be formatted, reduced or treated like any other ordinary text.

I am not familiar with the term "blind text". Can you explain what you mean, Ernst?
Thanks.
Also, can you attach the .afpub file that demonstrates the problem, so we can look for odd settings rather than having to guess and ask questions. That would be helpful.

I believe Filler text is being used. It does NOT behave like other text and should be deleted by clicking the checked menu Text - Insert flller text.

Have you looked at the Text Frame panel, Vertical Alignment, to see if the first icon on the left is highlighted?  That is "Top Align."   Also under the section "Baseline Grid" is the box "use baseline grid" unchecked?   That might help.

Did you by any chance import the text from another software programme?  Also have you clicked on TEXT - SHOW SPECIAL CHARACTERS in the menu bar to see if there is anything there?

@Chris_K
Is there any planned timeline for this? Are we talking months, years?