If the water level in your pond is about the same level as the apex of the siphon, then I think what is happening is that it just flows through the siphon at the same rate water is entering your pond. I think that is what is going on and that this is why it is not easy to get the siphon charged. The siphon should only charge when the water level is too high (but below the emergency spillway).
To charge it at this level and maintain the suction you may have to prevent the suction break from killing your suction. Normally the pond level is higher when the siphon charges. That higher water level will provide substantially more flow to the siphon than your current water level can. There is balance between the amount of water flow and the amount of air coming in through the suction break. Too much air coming in through the suction break could be making it difficult for you to charge the siphon at this pond level. Looking through your steps, it doesn't appear that you have tried preventing air from entering the siphon break. You may try closing it off on its end and then try all the steps you mentioned again. If there isn't an obstruction at the entrance of the siphon, then it should go to roaring (assuming suction isn't broken from the exit side due to lack of sufficient flow).
Word of caution, if this worked, the siphon would continue working until the pond is emptied to a point where air sucked into inlet of siphon breaks the suction ... however deep that may be ... unless you can easily open the suction break again to kill the suction. You might first verify that it is the suction break that is killing suction. Trying all the steps again, have an assistant observe what happens on the pond side at the suction break when you remove the plug. By your account, the open end of the suction break is below the surface of water (something I missed before). So, if it is breaking suction, a whirlpool will develop, and air will enter into the siphon killing the suction. Although this wouldn't verify the inlet is not obstructed, at least you would know if the suction break has been a problem for charging the siphon at the pond's current level. If the suction break didn't form a whirlpool, this would not necessarily mean your inlet is obstructed. Suction can break on the outlet side as well if the flow of water is not sufficient. At your pond's current level, it may not be possible to manual start the siphon due to the inability to initiate a sufficient flow of water.
It is always best to confirm proper working condition at the siphon's limits of pond levels. The pond level that initiates suction and the pond level where suction is broken. That's why I recommended allowing the pond to reach a level where the siphon should activate. Just want you to be safe without having unwelcome surprises. I'll close with this. The current status as you described it, is just what I would expect from a siphon that is working. It should let just enough flow through to balance the flow coming into the pond. It could be difficult to manually charge at this level. This of course doesn't mean that there is not something wrong. Not much help, I know, but you seem to have a good handle on how the siphon works and will be able to determine whether it is working or needs corrective action.
