Hi Combatistor,
while I don’t have exhaustive answers for all your LED queries, I have found out the following so far:
• LED_SEND and LED_RECEIVE macro has been replaced by LED_SEND_RECEIVE (with release of 0.9.7)
• System commands (ESP only) | OpenMQTTGateway v0.9.9
• Disable blue led on esp32-dev - #2 by DigiH
with me using the latter to avoid the constant blue LED blinking 
Hope this helps a bit.