I figured out a way to get the -15 into column (h) of Form 8949 / Line 10 of Schedule 😧 In the "Dispositions" input, you go to "Less Common Scenarios" and, under "Overrides", there is a "Total gain (loss) [Override]". If you put the -15 there, it will flow to column (h). That way, you can put the full line 9a amount (100) in the K-1 input and achieve the -15 offset through Form 8949!

The only remaining problem is that, when you do it this way, you still cannot get zeroes in columns (d) and (e) while preserving the "Q" and -15 in columns (f) and (g). You have to either:

1. Have all of columns (d)-(g) blank and just a -15 in column (h); OR

2. Put a 15 in column (d) and zero in column (e) (creating a +15 gain), so that then you can have the "Q" and -15 in columns (f) and (g). 

I chose option #1 with a description that says "K-1 Line 9A Gains excl by §1202 (Code Q)". That way, the Code Q is in there somewhere, and, hopefully, the entry makes sense.  I didn't like option #2 because it has actual numbers in the columns that don't add up.  Neither is perfect.  Intuit should let you override columns (f) and (g) as well, so that you can create an accurate entry.

@PhoebeRoberts : I welcome any further thoughts!