I'm pretty sure that there is absolutely no way of knowing how much Jason actually wrote for Trilogy tracks unless The Boys themselves tells it, but looking at solos it's pretty easy to compare what is Jasons (Stop When The Red Lights Flash) to Billies (Makeout Party, Dirty Rotten Bastards, Loss of Control, Let Yourself Go etc) and then there is of course Lady Cobra where both play their own solos. Then there are solos where you can't tell at all is Jason or Billie playing the solo (like Missing You) or other songs that I wouldn't know without seeing Cuatro (Carpe Diem and Sex Drugs and Violence - both from Billie).
But these are just boring assumptions; during Cuatro, Billie mentioned that Jason is on the right side of the speaker. Lets assume that if Jason is on the right, he also wrote it and vice versa with Billie. With this huge knowledge, we can tell exactly who played solo's in Trilogy and maybe tell also who played the riffs.
Before I begin, let me just say that this list is made with my own ears, they might lie to me but I'm pretty sure most of these are correct.
Let's start with Uno!
Nuclear Family: Jason (Intro riff + Rhythm), Billie (Lead + Solo)
Carpe Diem: Billie (Lead + Solo), Jason (Rhythm)
Stay the Night: Billie (Lead + Solo), Jason (Rhythm)
Let Yourself Go: Billie (Lead + Solo), Jason (Rhythm)
Kill the DJ: Jason (Rhythm), Billie (Lead + Solo)
Fell for You: Billie (Lead + Both solos), Jason (Rhythm)
Loss of Control: Billie (Lead + Solo), Jason (Rhythm)
Troublemaker: Jason (Lead and possibly solo), Billie (Rhythm)
Angel Blue: Jason (Lead + Solo), Billie (Rhythm)
Sweet 16: Jason (Rhythm), Billie (Lead + Solo)
Rusty James: ??? (For me it sounds like Jason is doing intro riff + both solos, but not sure at all)
Oh Love: Jason (Rhythm + First part of the solo), Billie (Lead + Second part of the solo)
Dos! (No wonder I love this album)
See You Tonight: Billie (Everything)
Fuck Time: Billie (Lead + Solo), Jason (Rhythm)
Stop When the Red Lights Flash: Jason (Lead + Solo), Billie (Rhythm)
Lazy Bones: Billie (Lead, Rhythm and Solo), Jason (Melody during 2nd verse and after 2nd chorus)
Wild One: Billie (Lead + both solos), Jason (Rhythm)
Stray Heart: Billie (Lead + Solo), Jason (Rhythm)
Makeout Party: Billie (Lead + Solo), Jason (Rhythm)
Ashley: ??? (Solo is from Billie but otherwise zero idea who is playing what because of those double-guitar parts)
Baby Eyes: Jason (Lead), Billie (Rhythm)
Lady Cobra: Billie (Lead + Solo), Jason (Rhythm + Solo)
Nightlife: Billie (Solo), Jason (Rhythm) - Can't tell is Billie playing anything else than solo in this song
Amy: Billie (Everything)
Tre!
Brutal Love: Jason (Lead), Billie (Rhythm + Solo)
Missing You: ??? (Absolutely no idea at all)
8th Avenue Serenade: Billie (Lead + Solo), Jason (Rhythm)
Drama Queen: Billie (Acoustic + Solo), Jason (Rhythm)
X-Kid: Jason (Lead + Solo), Billie (Rhythm) - I was very surprised hearing Jason doing the solo and not Billie
Sex, Drugs and Violence: Billie (Lead + Solo), Jason (Rhythm)
A Little Boy Named Train: Jason (Lead + Solo), Billie (Rhythm)
Amanda: Billie (Lead + Solo), Jason (Rhythm)
Walk Away: Billie (Lead + Solo), Jason (Rhythm)
Dirty Rotten Bastards: Billie (Lead + all 3 solos), Jason (Rhythm + 2nd Solo)
99 Revolutions: Jason (Lead + Solo), Billie (Rhythm)
The Forgotten: Billie (Piano), Jason (Solo)
There you have it. Does everyone else in GDC agree with this most accurate list of who plays what during Trilogy? Alsoalso, if you have ears, consider listening Missing You and telling me who is playing what 'cause my ears just flew off.