Just a quick response on this one (I will read other stuff carefully tonight).
Electra needs to parse and convert preset JSON data after it is received. During the transfer and parsing of the preset, MIDI processing is suspended. The reason is that until the moment that the preset is ready to use incoming MIDI data could not be applied anyways. The time needed for that depends on complexity of the preset. I would expect up to 1 sec for very complex presets - I mean this type of presets that @NewIgnis develops 
Page switches are different in this, they were designed not to interrupt or delay MIDI processing in any way. So my suggestion would be to use page switching instead of presets uploads and use devices to cover all required MIDI channels. Adding a SysEx command for switching pages to firmware should be fairly easy.