Thank you for the summary! Just ran into the same issue - I can transfer e.g. the Microwave XT preset from the public library, which is >120kB in one sysex message, allright using the Chrome app. But when I try to send the same message via JUCE app I have written or even via Midi-Ox, the Electra One will always display Preset Transfer failed, please retry.
I even looked at the Chromium source code of the WebMidi implementation, but they don’t do anything different than the JUCE Midi code, calling midiOutLongMsg of the Windows API, so I have absolutely no clue why it would work in Chrome and not in JUCE.
Anyway, splitting preset transfer into smaller chunks certainly will make less headache in the future.