Well you are a production engineer with special experience is quality control are you ?
The errant casting fin could be due to a problem with one of the moulds on a 100 mould casting machine most of which were picked up at the factory.
While is is a bummer whan it happens to you it is a cost thing.
Honda had the best quality control but Joe public will not pay the extra $ 500 that the better quality added to the price tag.
So yes it is a dollar thing and some one at Kohler had a long meeting with some one at Hustler and decided upon the course of actions to be followed.
Obviously it is a random problem affecting only a few engines and covered by warantee.
Oft in cases like this Hustler will get an extended warantee out of Kohler so they can sell their mowere with a longer engine warantee than their compeitiors.
In a perfect world this would not happen but in a perfect world you would happily pay double what you did for your mower so every one can be checked and will be perfect.
But this is not a prfect world, it is a commercial world most factories in the "West" are only marginally profitable at best and that profit get squeezed every year.
COld comfort for you and you are quite justifiably cranky but that looks to be more a problem generated by the dealer than the company.
Remember you are Hustlers customer, you bought Hustler mower, you are not Kohlers customer they did not sell the engine to you they sold it to Hustler.
Kohler have a big waranttee department but they do not talk to Joe Public, they will read your letters and emails but don;t expect a tech to spend $ 200 worth of his time talking to a person from whome Kohler made $ 50 profit.
They will happily read and file and even action something in writing becuse that is fast and cost efficient.
Back in your first post you might note I suggested you cantact Kohler in writing on paper or email.