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I read somewhere that suu ankou can be completed by a discard if it fills the pair (as in you have 4 concealed pons in your hand and a single wait ). Is this true or does it vary somehow or is it just a plain myth, some people told me that since suu ankou requires a concealed hand you can't win it with a ron - since the set that you call with ron is considered open.

Another question is about chombo with a wrong declaration of riichi. Is the round re-played if a player whos gone riichi with a no ten hand comes up on wash out ?

Also you get a dead hand if you show tiles from the dead wall or from an opponents hand but do you get a dead hand also if you knock off a tile from the wall or from your own hand ? Does it always require more than 5 tiles to be revealed for a chombo regardless of from where the tiles where revealed from ( dead wall, wall, own hand, opponents hand ) ?

The 3/4 concealed pons yaku simply refer to the pons being concealed, not the hand. As long as the relevant pons are all self drawn, it doesn't matter. But for Suu ankou the only part of the hand that doesn't have be concealed is just the pair. So yes, you can ron on the pair and only the pair for suuankou

Noten Riichi wouldn't cause a replay; just a penalty from the offending player.

by the way, a su anko tanki machi (waiting on the pair) is actually a double yakuman at most tables. so yea you can win by ron with a su anko hand and and you should be very happy about it.

also, accidentally knocking over tiles is almost never an automatic chombo. especially if playing among friends. when I play with friends, and someone knocks over the rinshan hai, usually we jsut switch it out with another tiles off the dead wall.
At a jansou, depends on the house rules, but you know, it happens, so in my experience its usually not a big deal.