[Slam]

Congrats on getting the car done! The belt whine may have more to do with the position of the balance belt idler. You're right in that the belt should actually be fairly loose, but that idler contributes a lot to the whine. The belt shouldn't be spinning it at all times. When revs increase and decrease the idler helps deflect some of the slack - if you watch it in action it's kinda fascinating. Try playing with and see what difference that makes. New belts will whine, too. I use the krikit and average seven readings to get a figure.

Seems your temps are now close to where they should be, odd as that sounds. So many of us are used to 944s that run hot and worry about anything lower than halfway on the gauge. After I did the head and swapped the rad on my 944 the temp gauge almost never gets up to halfway. My 924 gauge behaves the way you're describing yours.

Detonation I don't really have ideas about. Have you swapped in a DME without the chip? Do you have vacuum leaks? Maybe the Hall sensor isn't hearing the pinging.