Centering the dot would indeed keep you on the 3.75 degree slope, and if the stack height we are assuming (100 AGL) is correct it would be at 4261 MSL (give or take a couple feet). The 3.75 degree slope would clear the tower by about 93 feet by my calculations. That's not much but it would clear.
I don't think the diamond "thingy" would have instantly jumped to the bottom at JAMID if you did not increase descent rate -- it would start moving towards bottom, but how fast? (see below)
In the photos of the damaged tower, it appears the a/c impact was about 15 feet down from the top, so that would put the a/c (93+15) or 108 ft below the desired path. How many dots is that on a G1000? I don't know, but experiments with some Garmin trainers (e.g. GNX 375, GTN 750) showed that one dot is about 80 feet at any location between JAMID and RW20. The 108 ft error would then be displayed as (108/80) or about 1-1/3 dots.
With a 90kt ground speed, descent rate prior to JAMID would be 522 fpm and 597 fpm after JAMID -- an increase of 75 fpm. So if you did not increase descent rate at JAMID, you would be 75 ft high after one minute, so that's just shy of one dot. So, if the above math is correct, the "thingy" would not jump to the bottom -- it would take a full minute to show one dot of error. In a perfect world, that is.
In the real world, manually holding the descent rate that steady (e.g. 522 plus or minus a couple fpm) in actual weather in a small a/c would be somewhere between difficult and impossible, no?
The big assumption here is that the G1000 is scaled so one dot too high or low is 80 feet. Anyone have a G1000 simulator to run the experiment with?