(1) Absolutely, things don't always go right. I always ensure I have a recent Windows system image backup before I do anything big. And it is good to advise (as you reminded me) to ensure an over-the-top install will not wipe files when that is not wanted.
(2) I recall 1909 came in through Windows Update as an optional update. I researched it a tad & saw a Microsoft blog that said it would basically just turn on switches to transform 1903 into 1909, just as you say. Indeed the install took 2 or 3 minutes including its reboot to complete.
(3) I found an old thread in which I said it was 1903 that I installed over itself. Apparently, when it first came out, the MediaCreationTool (MCT) fetched OS Build 18362.30. As an .ISO, it worked to upgrade 1809>1903. On a USB, it would not upgrade 1809>1903. It gave an error message: "This PC can't be upgraded to Windows 10"...
But when the MCT fetched 18362.175, that did work to do the over-install 1903>1903 from USB installation media. That was back in July, 2019. As you know, one must run DISM on Install.esd to know the OS Build...
"DISM /Get-WimInfo /WimFile:E:\Sources\Install.esd /Index:1"
(4) But looking into C:$Windows.~WS\Sources\Panther\SetupAct.log (which has survived Disk Cleanup), going by what you said (which makes sense to me), it looks like I later also over-installed 1909>1909...
2019-11-18 22:56:48, Info MOUPG *************** SetupHost Logging Begin ***************
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: ModulePath = [C:$Windows.~WS\Sources]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: WorkingPath = [C:$Windows.~WS\Sources]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: LoggingPath = [C:$Windows.~WS\Sources\Panther]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: MediaPath = []
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: InstallFilePath = []
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: ActionListFilePath = []
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: CmdLine = [/Download /Web ]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: Mode = [0x8]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: Scenario = [0x7]
2019-11-18 22:56:48, Info MOUPG SetupHost::Initialize: Flags = [0x800]
2019-11-18 22:56:48, Info MOUPG SetupHost: Setup build version is: 10.0.18362.418 (19h1_release_svc_prod1.191005-1654)
2019-11-18 22:56:48, Info CONX pGetHostCSDBuildNumber: 'HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\CSDBuildNumber' does not exist. Assume 0
2019-11-18 22:56:48, Info CONX Host OS version: 10.0.18363.0 (0.0) Platform 0x2, SuiteMask 0x300, ProdType 0x1
2019-11-18 22:56:48, Info CONX Source OS: Host SKU Info: Edition = 'Core', Edition Type = 'Client', Installation Type = 'Complete', Product Name = 'Windows 10 Home', Arch = amd64, StationType = Client Workstation, Stage type = Staged Image
2019-11-18 22:56:49, Info MOUPG Host OS License State [ Genuine ]
2019-11-18 22:56:49, Info MOUPG Host OS Architecture [ x64 ]
2019-11-18 22:56:49, Info MOUPG Host OS Language Id [ 1033 ]
2019-11-18 22:56:49, Info MOUPG User UI Language Id [ 1033 ]
2019-11-18 22:56:49, Info MOUPG Host OS Edition [ Core ]
2019-11-18 22:56:49, Info MOUPG Host OS Version [ 10.0.18363 ]
2019-11-18 22:56:49, Info MOUPG Host OS Build String [ 18362.1.amd64fre.19h1_release.190318-1202 ]
… snip ...
2019-11-18 22:56:50, Info CONX Target OS version: 10.0.18362.418