pearlm30, special thanks for the update and confirmation.
I note that if you select 1 the car will drive.
If you select 2 the car will drive and shift from 1 to 2 automatically.
Only when a speed of about 30 MPH is reached and D is selected, will shifting progress through 3 and into D and then all is normal.
Therefore you confirm that first is engaging correctly when selected via the shifter. Also that first is engaging correctly when second is selected from rest, so that first is engaged automatically to be followed by second.
I am not sorry to say that there is no indication that the one way clutch is slipping. What is more at this stage there is no indication that the transmission will have to be removed.
The symptoms as you have reported, at this stage do not look ominous. My original diagnosis regarding possible causes stands.
As a matter of interest, two arrangements are provided for first speed, such that manual selection of first, specifically provides engine braking for tricky situations.