Tim
I have now had the chance to read through your post and I hope you don't mind me having split it up a bit to make it easier to read. I have not changed any of the words.
I think there is one important point to note about the changes in 2019. All the code is fully compliant with HTML5 and CSS3 rules. That means that some things that were done in a proprietary way before have had to change. I cannot say that is the cause of any of your issues but it could be. A very important point though is that because of this compliance, you can do anything that is within those rules and that may not be true of other authoring tools.
Looking at what you report, it is clear this needs to be seen by Adobe and I will be asking them to look at this thread. I think what they will need is a copy of a project where you have added some notes pointing out the issues in relevant topics. Alternatively create a project with just a few topics to illustrate the issues. Send it to me as described on the Contact Page on my site and I will pass it all on.
I am not one to start authoring tool wars as they all have their pros and cons and one will suit some users while another will suit others. I would be interested to learn privately how you get on with Flare. You can email me via my site.
See www.grainge.org for free RoboHelp and Authoring information.

@petergrainge
Hi Tim,
As per my understanding of your post, here is the breakdown of things (I might have missed one or two points, please let me know):
- Regarding the number of levels that can be defined in RH 2019, yes, it supports up to 6 levels. However we can help you with the 7th level style customization if you are still using it in your project.
- Conversion of '<x>.' to '<x>:' (Dot to Colon) in Formatting - Please share a sample Classic project, which illustrates this problem on upgrade. We will investigate it and will share our findings.
- Mixed usage of multiple List Styles is not allowed in RH 2019 as the newer design is fully based on CSS3 standard. It is not feasible to apply multiple styles at different levels of the list. As a workaround, create a Multilevel List Style with such a formatting defined on different levels.
- Expand All/Collapse All is not present in RH Classic Responsive HTML5 layouts out-of-the-box. It is added to RH 2019 HTML5 Skins. You can try it out in RH 2019.
- The numbering continuing in a different list in same topic (with same multilevel style applied), is explicitly done during Classic Project Upgrade. In Classic, by default, if you apply same list style in different lists in the same topic, it results in numbering continuation. To maintain that it is done in project Upgrade. In case it is not working same on your side as I suggested, please share a sample classic project illustrating this issue. We will investigate it.
Please let me know in case I missed anything. I have sent a private message where you can share the Classic projects with us for further investigation.
Regards,
Vikas Singla