Please elaborate more on what type of solution you have - in general we cannot migrate or deploy the custom visual studio solution to SharePoint online, we need re-develop the solution using the modern technology SharePoint Framework (SPFx).
There are couple of things we might look around for this fix.
This error usually occurs when the Windows Installer cache files (C:\Windows\Installer) are missing/corrupted. To further isolate the issue, you can try running ROISCAN on the machine and see how many installer cache files are missing. If these cache files were deleted manually ...
Yes, this is possible by deploying and configuring Office Online Server.
Details can be read here: https://docs.microsoft.com/en-us/officeonlineserver/office-online-server
Instructions are for SharePoint 2016 but applies for SharePoint 2019 as well.
Yes, we could still audit views of Documents in SP2019.
Although the "Opening/Downloading" option has been REMOVED in SP 2019 audit log settings, the option is still available in site settings-> Audit log reports-> Run a custom report.