How to achieve factory-reset or bootloader mode for Flywheel (Glorystar Nebula rf3288) tablet without buttons?

I've gotten stuck trying to upgrade a Glorystar Nebula 15 kiosk Android tablet (as customized for the orphaned Flywheel spin-bikes) to a vendor-provided **single-file** Android 7.1 firmware image (7.20191202.115231.img, 715MB). A special challenge here: this tablet has **no physical buttons whatsever**: no power, no volume controls, no home. (It simply turns on whenever power is supplied; there's a click power toggle inline on the DC power cord plugging into the back. It's got no battery for unplugged use.) Following the vendor's instructions (using a Windows AndroidTool program 2.35), it was able to 'Switch' to detecting a LOADER device, and appeared to write firmware - but after a lack of progress ending on a log line mentioning 'Success', I disconnected & power-cycled in an attempt to view my hopefully-fresh-new-stock-Android7.1 tablet - and instead found it in a state where it never proceeds past a 'GS' logo boot screen. Currently, from this GS screen: * the tablet only ever appears as "Found One ADB Device" in the Windows AndroidTool, whereas previously it could 'Switch' to "LOADER" * it can be detected via adb from MacOS – but oddly, not via adb inside the same Windows10 VM for which AndroidTool was sorta-working.and, adb shell commands give a error about the sh-path not being valid. * adb can reboot it into recovery mode - but without physical buttons, no navigation/selection from the recovery menu is yet possible, and it does not appear to adb or AndroidTool in this mode * adb can seemingly also reboot it into bootloader or fastboot mode; in this mode it no longer appears to MacOS adb, but does appear in MacOS fastboot devices list. However, attempting to connect the tablet while in this mode to Windows AndroidTool still only ever shows "Found One ADB Device" (or nothing) I'm looking for any path out of this dead-end state that allows a re-attempt of the flashing the original vendor single-file firmware IMG, or other potential Android 7.1 firmwares. For example: * I've found a header on the motherboard with pins GND / PWR / HOME / ESC / V+ / V- / MENU / K2 / K1. However, hand-shorting PWR or V+ to GND with a small wire (alone or together, at the 'recovery' menu that's supposedly looking for vol-nav inputs or other times), has had no visible effect since the tablet's been in this state. * Can the vendor's single IMG be broken into the smaller more-specific parts that fastboot seems limited to flashing? Can applying those, or some other, smaller steps restore a true "Found One LOADER Device" mode, such that the AndroidTool might be able to make another attempt? Etc – open to whatever gets Android 7.1 on this device, including, at this point, all-or-nothing gambits to resuscitate or brick it. Any ideas?