should it not be visible?
No because it would be inside the engine either in the cylinder liners or cylinder head.
can bad (automotive) coolant cause this?
But having thought more about it, you could have damaged a cooling pipe somewhere, that would cause the flow of coolant to slow down and increasing the coolant temperature.
I don't know these bikes intimately, but follow the all the pipes and hoses and see if any are kinked, squashed, or buckled.
Also taking some pics would help, I'm sure you have a cellphone with a camera?
How do you know the coolant in the reservoir is boiling? Are you absolutely sure it's not just air bubbles? I ask that because I can't see how a remote reservoir could get enough heat to do that. The coolant in the reservoir would have to be constantly circulating in order to boil, and that's the whole point of the reservoir - it catches coolant which overflows once the coolant in the engine expands due to heat, then allows it back into the system once it cools down again. It's not part of the pressurised, flowing system and should remain relatively cool, hot maybe, but never so hot as to boil. To boil it would have to be well over 100 °C!
Stick a thermometer in it to see.