My friend, you are so big brained. My immediate first thought is that I completely agree with you!!! Colin is too much of a considerate man to go spelunking in her secret passageway with two fingers, however, upon rewatching the scene, there are definitely two '*OH!*' moments from her, indicating that I believe he STARTED with his middle finger and then got assistance from another!
I argue the wingman was his index finger
His index and middle finger are much closer in size and length than his index and ring finger. In proper cave diving, it is important for equipment to be consistent! His ring also maintains the same orientation pre-fingering as post-fingering, and with how heavy they were going at it
that thang would have been on backwards by the time he was done
However, you are so right-- he was using the heel of his palm to rub her off whilst he was (heavy) petting her from the inside out!!! Which is why he feels alright to adjust her dress with his thumb. He literally had her in the palm of his hand.
The two in the pink are his index and middle. Penelope is a woman who wants it all and he's giving it to her!