...no I would not accept it by word of mouth. The Flying Thunder God Technique is an S rank move. The only proof I would want is, WHO did you learn it from...
The only way to get that information across is word-of-mouth. The "master" of the technique can be redefined, and has been in the past. Hiraishin is just an example. Tomi claimed to be the creater of the technique and all that, but by the time he was RPing as such, I am quite certain that there had been some others out there who had similar claims at the time.
In the end though, his hiraishin legacy is the one that carries the most weight.
... The argument about Izanagi happened because some guy battering rammed into my rp and decided to kill me in the midst of me using the jutsu. There would not have been an issue if the rp had go on without him. I seem to frequently have issues with people refusing to talk to me about wanting to join an rp. You'd be amazed how many problems are alleviated when you just set up how the rp is going to go beforehand and not just have people kicking the door down willy-nilly...
The argument/discussion happened primarily because the timeframe for Izanagi was incredibly hard to pinpoint when an action that took place outside the zone, without referencing the actions that occurred in the zone. That was the initial issue, but this would later on evolve into just how long Izanagi actually lasts, and there is where Rinha would like some definition at. As it is now, it is largely ambiguous just how long Izanagi can last, because the set time for the most optimal conditions is anywhere between 1-10 minutes (or something like that, I and others figured those numbers out in the actual thread more accurately) or even post-mortem if there is a transcript Izanagi going on.
Initially yes, someone butting into the fight without bothering to reference the fight caused the discussion to start. However, how long Izanagi can last did become a question at one point, and was a cause for further arguments/discussions.