Using Geometric Theme, RapidMaps displays in preview mode (but does not have all functionality - to get all functionality, publish the page).
RE: Add Pin - this is for adding locations manually (Long/Lat/Name, etc)
RE: Ghost Pins - this is because of the use of add pin and trying to use search field at the same time.
To properly add an address, you simply type it in the search field and hit enter (no ghost button generated)
RE: 6.3.7 and 6.3.8 concurrently - this is possibly an issue as they would likely access the same stacks, plugins, etc and cause code conflicts imho
RE: 6.3.8 wonky behaviour - not my experience, should do a full install as per the Announcement thread if you have not done so already.
I have a demo project that works perfectly in RW 6.3.8, if you want it, let me know.