I fully respect the playing-it-safe from game designers. Having a product that does not upset the customers is important. That being players not feeling ripped for their ball in play (or worse). And operators feeling they have a stabil product with lesser service needs.
However, I would like to see them a little gut’tier. Meaning, if the game is set for competition play, expect every switch to work and start the ball saver (go valid if you will) on first switch seen out of the shooter lane. Including the trough.*
That being said, I do not believe one should go that far, as to start the ball saver and goto auto-plunge reling only on the main shooter lane switch (to prevent forever short plunge attempts). Under any circumstance. That is just too dangerous. With the risk of the game starting to auto plunge in competition before the player has stepped up to the game.
If a no-forever-short-plunge-attempts rule has to be applied, there is not way around two shooter lane switches the way I see it. With one near the short plunge exit.
Edit. Forgot to state, that this reasoning is based on the logic of a trough that is not filled up. So you can rule out balls entering from the shooter lane exit.