Definitely at least 4-6 in Lei Fang's favor. Bass' speed is God awful and she has an enormous gain on him when she parries him or when he is unsafe, his crushes are extremely few and seldom useful against her (or anyone for that matter), she can parry on Bass' backturning throw combos (parry while back turned), she crushes him very easily making it very hard to use his OHs aside from low OHs, 1P+K is damn near unpunishable, her OHs and OH type strikes shut down his striking game when she isn't parrying, his options are limited to playing extremely defensive against a defensive character and wait for the golden opportunity to throw punish or go for a pickup. She doesn't need to break defenses, all she needs to do is just chill.
Biggest thing against her in this is her weight vs. his. Sure she has to modify combos but why combo when you can just tear through just about everything he has?
Leifang has more to lose while using her parries than Bass has when getting parried. A parry will give her what exactly..? Her unshu will give her a deep stun, which is the same for everyone, and her mid parry will give her 9KK 46P for free, just like against everyone. There is no additional disadvantage for Bass here, especially when he's heavier. There is however, a bigger disadvantage for Leifang for using them. If Bass grabs one of those parries, it's bye bye half health. Yeah, we do know Leifang can parry everything. But it's a huge risk to parry against Bass, more so than against anyone else in the game, except maybe Tina and Bayman. Without her defensive tools, Leifang is pretty garbage. Combine that with her crappy offense, and we have a stale mate. (In B4 'she doesn't have crappy offense').
The examples you gave aren't exactly convincing. Here's one.
"she can parry on Bass' backturning throw combos (parry while back turned)"
Leifang can parry Bass when BT, but Bass can also grab or OH her. And why would you use that grab out of the huge list of grabs in the first place, if you know she can do that? It's a 50/50 guess anyway. Bass gets more grab damage than Leifang gets for parry damage. No advantage for Leifang here.
Explain to me why she doesn't need to break defenses. What exactly can she do to just chill? What are these godly moves that Leifang has that leaves bass at such a huge disadvantage? If Bass just stands there and blocks, waiting for an unshu or a chance to grab, Leifang can't do much. Her attacks are not gonna get her anywhere, unless the Bass player makes a mistake. She needs to use either grabs or OHs to get anything respectable by chipping away damage. With his neutral 4 frame grab, he can punish unshu every single time if he pays attention. She relies on counter hits to get anything going, and if the Bass player barely attacks, she's simply standing against a big rock. If you're trying to spam attacks with Bass you're doing it wrong. Just because the Bass player can't attack like he's playing with Christie, doesn't mean that it's a very bad matchup for him.
Bass needs to be careful while attacking, but Leifang needs to be very careful when using her best tools. It's not an advantage for him, but the advantage Leifang has is not as big as people say it is. I invite any good Leifang player to play any good Bass player, to see of you'd just chill.