Whether Omise charges a fee when an authorized (not captured) transaction is reversed?

No. Omise will charge a fee (3.65% + VAT) only when the transaction is successfully captured and settled by the bank.
So if you afraid of refunds, then you can set up my extension to preauthorize the payment amount during checkout, and then you can manually review the order and manually capture the payment.
This process is described in the following demo videos: