You're correct to say that Corbyn has called for ceasefires - but he's only ever asked for things to be done via the UN where Russia exercises a veto (they issued, I think their 13th veto on Syria related matters a few days ago) so this calls are ultimately toothless. He knows they won't do anything but it gives him the cover of plausible deniability, and gives yourself a little nugget to throw around in internet arguments. Try and find something he's said that'd lead to substantative active in Syria and then compare that to his statements about Israel, KSA etc. It's pretty clear what his basic point of view is. The only time he ever got exercised was when Trump bombed an empty airbase.
I was involved in a picket outside the STW conference a few years ago, organised by Syria Solidarity. I remember talking to one guy who was super keen to denounce us as warmongers and I remember a friend asking him what his solution would be. He said "well, all sides should withdraw". Like Russia would just pull out, retire from the field like honest gentleman. I realised then that he didn't actually give a damn about Syrian lives, they didn't even factor into his thinking. It was all about enacting his post-Iraq rage at our establishment. I'm certain this guy would be known to JC and very probably on first name terms, and I'm sure the worldview would be pretty much the same.
I don't want to ruin my evening arguing about this stuff so I'll leave is here. It's a video of Corbyn siding with Russia and Assad in wake of the chemical attacks in Syria in 2013. This was the Ghouta massacre which killed 1400 people and violated Obama's "red lines". Assad did it, yet Corbyn here is happy to parrot Russian propaganda, a classic "useful idiot". The story is the rebels gassed one of their own towns in an attempt to create a "false flag". It's stuff like this that makes him unfit to be PM in my view, not any purported tax increases for the rich.
They really aren't.
That just popped up in my timeline funny enough. A 9/11 Truther and Assadist invited to speak to a CLP.
This may not be characterise of mainstream Labour grassroots opinion. You would know better than me. But I think the pass this stuff is given by Corbyn, Milne et al and the embattled way that the Corbyn supporters dismiss any criticisms against him as plot (MSM, Blairities, take your pick) lends this stuff legitimacy. I'm pretty sure that you'll be more tempted to side with Corbyn and co. than me, and are already mentally rehearsing a litany of rebuttals, maybe your mouse finger is twitching looking for counter-arguments. If you fully commit to it, you can convince yourself ithe whole war and revolution is was dreamt up by M15 before you go to bed.
People trust these people, have their hopes for the future pinned on them. and as you have evidenced upthread are totally willing to dismiss competing claims on this emotional basis. People will contort themselves like mad to convince themselves the terrible, terrible positions that Corbyn et al hold are legitimate. That's why I don't want him in No 10.