First, frequency progression occurs because the progressive length difference in clubs and the progressive head weight difference as you move from the #3 iron to the wedge. The frequency has to progress 3-4 cycles higher as the clubs get shorter to make the clubs feel and play the same at the different lengths and head weights. If all the clubs were one length and all the head weights were the same, then yes, the frequency of each club should be the same.
As far a Tour players playing wedges softer, some do and some don’t. The main reason some do, and I happen to agree with it, is that wedges are rarely swung full. 3/4, 1/2 swings are more the norm, so there is not as much force applied. Also, the slightly softer flex provides more feel around the greens. This concept is something that Dave Pelz advocated back in the day when I worked with him. At that time, most of the players he worked with played slightly softer shafts in their SW’s and LW’s.