While I agree I see personal bias in what he writes on Twitter, I'd hardly call it pro-rebel. Anti-government (more like anti-syrian regime), sure. It show in the choice of topics. But the question is how much it shows in the analysis itself.
As for Hexamine/Sarin attack/Postol issue: Hexamine was found in Sarin attacks by OPCW, it was declared by Syria to OPCW. Postol is not a chemist, nor CW expert, nor on the ground, nor very credible wrt his claims about Syrian chemical attacks, as several of his claims were already rejected by OPCW, who found Syrian regime responsible for the April 4 2017 attack (Postol had all kinds of theories about this attack). I prefer OPCW's conculsions to some random profesor's taking sides and judging from a distance.
https://www.armscontrol.org/factsheets/Timeline-of-Syrian-Ch...
You quoted Partisangirl, and I just can't take her seriously as an unbiased source either. Just one recent example of clear bias: Despite big civilian protests last few weeks in Idlib, she only tweeted one picture of some marching jihadists. She's very clearly pro assad-regime, at the expense of truth, accuracy, and peoples lives for that matter.
So yeah, Higgins is biased, but quoting other biased, non-expert sources doesn't really improve the matters.
Re: fake sniper. The experiment can be both revealing of reporting bias and highly irresponsible. People criticized the irresponsibility angle at the time. Witch hunt seems to be your interpretation.