I can answer the question about how common the $10 fee for non-members in events is:
year count
2014 174
2015 321
2016 298
2017 413
2018 454
2019 406
2020 278
2021 368
2022 439
2023 594
(10 rows)
So far in 2023 there have been 447,982 players in rated events, so the use of the $10 fee represents about 0.13% of the players in those events.
The US Chess office does monitor the use of the $10 fee to watch for abuses of it.