I cannot comment on your first observation, as I've never used that functionality. However, I perform renaming all the time with PF8.2 as I always have and as you describe: two (2) successive clicks and I can type away. When completed, I hit the 'Enter' key.
What version of PF8 are you using that no longer provides this behavior?
Presently 8.3, but it was the same with 8.2 and if i remember well with 8.1.
Please note that my remark is a statistic remark... meaning sometimes it might work with 2 successive clicks, but most of the time it doesn't and i then have to use the Enter key to underline the name of the file
Okay, now I understand. Funny, because I can't recall ever having to go the 'Enter' key route to make renaming work. I would expect that some of it could be due to the settings of the mouse (click- and tracking-speed) in the Apple preferences.
I had noticed a tendency under Mojave to reacting to the two (2) successive mouse clicks as a double click and opening the file. I had to adjust the mouse as mentioned above to fix this.
I also see a tendency of PF8.2 to react very slowly to the successive mouse clicks. (I can't speak to PF8.3 as that release is so bug-infested I stayed far, far away from it.