Regarding descending, I think Sir Instrucalot has it right, in that pulling the throttle back, or lets call it reducing power, is the correct thing to do, just not TOO far back, which it seems happened here due to leaving their descent too late. When commencing a descent, its poor airmanship to leave the cruise power setting and simply 'shove' the nose down in a fixed pitch prop C150, you'll end up with the ASI in the yellow arc most likely. Better to reduce power to say 2000RPM, lower the nose slightly and set up a descent rate between 300-500fpm. Of course if you've left your descent too late....well...you'll be needing a lower power setting which will put you past the green arc on the tacho, so best you pull the carby heat knob out!
Why not just properly, whilst still in the cruise, calculate a TOD point using a 500FPM rate of descent and upon reaching it, set the descent whilst maintaining cruise RPM?
That is all I have even done in any a/c with a fixed pitch prop!
Never ever had any problem with the ASI getting too high.