You are going in the right direction on the rate evaluations. I don't have time at the moment to double-check your cover 3 analysis as to exactly when international rate/fee changes occurred, but might it be
4.85 registered fee
1.10 return receipt fee, and
1.00 for letter between a half and a full ounce
Yes, the judges will be looking for in-period commercial uses. And you will want to locate a solo 3-ounce cover since that was the intended purpose of the stamp. It would be a glaring omission not to have one..
Also, I don't have the reference in front of me, but there was a good article in the "Philatelic Exhibitor" about 15-20 years ago about the weighing the quality of various uses. Specifically it discussed two general rules of thumb the author used to evaluate covers for purchase and exhibiting:
1. The percent of the subject-stamps on the cover.
2. The percent of the postage/fees paid by the subject-stamp.
In other words, better covers would have the subject-stamp be a high percent of the total number of stamps and the total value.
Cover 1: 2/4=50% of the stamps. 130/152=85% of the franking.
Cover 2: 1/3=33% of the stamps. 65/645=10% of the franking.
Cover 3: 1/5=20% of the stamps. 65/695=9% of the franking.
Cover 4: 1/2=50% of the stamps. 65/140=46% of the franking.
Cover 5: 2/2=100% of the stamps. 130/130=100% of the franking.
Cover 6: 1/3=33% of the stamps. 65/115=56% of the franking.
And naturally, a solo use would score 100% and 100%.
You can set your own acceptability threshold (or not). Clearly, some of these covers show the use of the subject-stamp better than others. And there are other factors to consider of course.
Add: 2 images digitally cropped from 9x12 flats, domestic rates, both paid exactly with a minimum number of current definitives. Philatelic? Both are addressed to stamp collectors.
February 28, 1990. 4 ounce flat during the 25 cent rate period (25+20+20+20=85)
March 6, 1991. 7 ounce flat during the 29 cent rate period (29+six 23's=1.67)
And add 3 solo uses:
4-bar handstamp with the bar portion held onto the base with a rubber band creating a vertical line across the bars.
Parcel post roller cancel from a sectional center facility.
Flats machine cancel: