I have recently seen a fan-theory that in The Force Awakens,
was the one that activated Kylo Ren's lightsaber, with suicidal effect.
Is there a reliable (preferably canon) explanation of whether this fan-theory is true or false?
The film's official novelisation categorically disproves this theory
The junior novelisation contains a very slightly different view of the event.
As does the film's script
as does The Force Awakens: Rey's Story
As I recall, right before the fatality, Han was telling Kylo to come home, and offered to help, and that this help could be "anything". This was an appropriate offer, because Kylo had a lot of baggage that would need to be handled. Han's son took the available offer as an offer to help the son accomplish whatever the son decided he needed. Prior to the lethal extension, Han probably thought Kylo's responses were seeming rather promising for Han's wishes, which were to follow the orders of Kylos' mother: bring him home.
If Han was to fail in his mission to bring his son home that day, his fallback preference would probably have been to wait another day, and pursue Kylo again at a later time. Suiciding would be to giving up on that goal, which does not seem to be what Han was doing.
So, that fan-theory seems false, based just on my interpretation of the content of the movie.