Oh, maybe the lack of recovery partition is my real problem, because I definitely deleted mine. The installer isn't really clear why it's failing, it just does its thing for a bit and then reboots into Windows 10 and says it didn't work, no explanation.
Things like this iPXE have a similarity to multibooting to where you may be a lot better off NOT having a (single) dedicated Recovery partition at all, but you need full responsiveness to Windows Update.
And please no hibernation for multibooting. POWERCFG /H OFF.
For recovery manipulation you need to utilize the REAGENTC command, and take advantage of the existing default C:\Recovery folder. Normally this is not considered the best location for "recovery" since if C: gets borked so does default recovery console.
And this really gives an example of essential use of inbuilt Helps.
On a mainstream system all you have is a single (hidden) ESP volume containing its EFI folder:
In admin CMD, type Bcdedit /Enum All to show all your boot entries.
If "recovery" is even set up, there will be a Boot Loader entry where the Device is a Ramdisk. If so there will also be a volume letter in square brackets if the target volume is unhidden at the time.
But for those of you with no recovery set up, you get to do it yourself as you see fit.
Type Reagentc /info, it will confirm recovery is disabled.
Reagentc /? gives you the next clue, the example /setreimage.
Copy the displayed REAGENTC.EXE /setreimage /? from the display down to the active command line (or just type it in). You get two syntax examples, it's the second one. But you probably don't even have an R: volume and you want to use C: anyway to make Windows more self-sufficient from its own partition.
After this operation is successful, there is still no apparent change under Reagentc /info, recovery is still disabled. And your BCD remains unchanged with no ramdisk object yet.
"Reagentc /enable" will then autoadd the ramdisk recovery bootentry to the BCD, directing the recovery routine (when triggered) to point to your C: volume which will always exist accessibly, plus most likely have enough free drive space to accommodate the larger updates to the recovery WIM files themselves, which are recently giving users show-stoppers when their dedicated factory recovery partitions were not big enough for that.
Now for the good news.
Once you find a ramdisk recovery bootentry in your BCD, an easy baby-step toward multibooting is a piece of cake. Everybody already has a Windows boot menu that you are using every time you boot, it is simply not displayed if there is only one entry, like most factory set ups.
So add the Recovery Console to the menu your dang self.
It can come in really handy.
From the Bcdedit output text, copy the shown GUID for the main ramdisk recovery object, so you can paste afterward and don't have to type it all into the following command:
bcdedit /displayorder {your-guid-here} /addlast
Then if you don't want to maintain the default 30 second timeout before progressing to the default OS on the bootmenu, set it for something like 5 seconds:
bcdedit /timeout 5
That's it.
Now if you don't have a touchscreen, you probably would be better off with the "legacy" text menu, for that you would need to change the bootmenupolicy from Standard to Legacy for the GUIDs that have a bootmenupolicy. Bcdedit /set {target-guid-here} bootmenupolicy Legacy.
Also remember if using Powershell it's still having trouble with the curly brackets so in every case they need to be quoted like Bcdedit /set "{target-guid-here}" bootmenupolicy Legacy.
