Tom Smallwood is the perfect example of allowing some control of the non-delivery hand if one wishes to not use their thumb. The non-delivery hand must be removed from the ball once the downswing passes the swing side hip.
I wouldn't allow the "chest throwing" at all.
We had a monthly sport pattern league that ran it's course for about 7 years, shortly after sport shots first started. We couldn't get any younger bowlers to bowl it. It would fill the center, with a waiting list at one point. Ran it's course, as all things do.
I have run a sport league for coming up on about 12 or so years now. Last year, was the first year we even had a bowler less than 30 years old (I was under 30 when I started the league). The team disbanded for this year, and only one player remained, which we are still trying to find a bowler to fill his team out. We have averaged 10-11 teams. Low of 9, and high of 12. Trios. This year, we have a few kids just out of juniors bowling it, as it is their first adult league. Side note, we have had our youth program bowling on USBC white patterns for the last 10 or so years, so they are used to having at least the USBC minimal amount of oil on the gutters.
Clearly, the demographics in your town are different than mine. But I do stand behind the overall numbers that have, as a whole member body, supported sport bowling and league base. I wish I could find the page I found those statistics, as I like to have proof to support my claims.
Even Simonsen's rev rate, although less then most of the two handers, is still higher than the average bowlers rev rate.
As a side note on Simonsen. GREAT person, and the sport of bowling needs more people like him in the industry. I wish him all the success he can attain.