This isn’t a complete answer to your questions, but this is what the help files in Paratext 8 say:
Paratext automatically marks certain points in the project history, including the following:
- Immediately before and immediately after a Find and Replace has been performed.
- When you set up the project to be shared.
- Just before you revert one or more books.
The following are other times when you might mark a point in the history of your project.
- Before and after making major changes to the text (for example, correcting spelling, or checking biblical terms).
- At significant and memorable points in the project (for example, before a consultant reviews a book, and also after you make changes to the book based on the feedback from the consultant).
- You could even mark a point in history before beginning your work every day as a reference point for each day’s work.
From my own observations:
In Paratext 8, when I looked at Project > Project History, it seems that I can see all send/receives that have changes, and I can click on any books that have changes to jump to them in the compare window. Alternatively, I can click on Tools > Compare Texts, which lets me compare the current text to any version of the text in the past. In the Compare Texts window, you can also see the various stages of the translation, so if you use the Planning tool you can jump to certain points, for example, just after a Consultant Check.
Doing a manual “Mark Point in Project History” simply creates another history point which has a description, as far as I can tell. While it isn’t always easy to find these, it does help you remember those milestones which might be difficult with only dates shown.