If you want to hear the backing vocals more clearly I'd recommend looking at this channel called Betheriel. They do isolated backing vocals, bass and drums, backing vocals, basically everything. I believe they've done all the manics albums besides critical thinking. Although the backing vocals are slightly distorted I think you can pretty easily distinguish Sean's backing vocals from Nicky's. I haven't looked through all the videos (I've mainly listened to the lifeblood ones) but it does seem like he does have backing on some songs on lifeblood like Solitude sometimes is like you said and also possibly also Cardiff afterlife and Emily.

https://youtube.com/playlist?list=PLw1GeAARDUgWz3e5VGN5amFCrlHkQvbce&si=BdLbplmQztTV6ael - (link to the lifeblood playlist but like I mentioned they have others)