I think the whole BJ "not Britney" is a bit OTT.
Seriously, it's clear Britney didn't want to record an album. Her label wanted an album for Vegas. Britney wanted to just do Vegas.
It's very clear that Britney sings on every track. However, at most it's a "He About To Lose Me" situation where Britney sings once, and the rest is done by Myah.
It's clear that she sang once, she walked in, she walked out. Done. Apart from a few songs, she wasn't inspired and did the bare minimum.
---
To say Britney "didn't sing" BJ is pure bollocks. She just sang the main vocal, no backings. And myah did the rest. Britney's voice is very thin, so if she chose not to do backings, it's no wonder that you hear Myah more.
But there is NO proof, that Myah is the only one singing.
On all the Myah "vocal leaks" there is still vocals missing from the final version, which are Britneys.
B stans are so dramatic