I've never used any sealant on O Rings, although as said, a bit of spit helps to slide them in if it's a push fit condition.
I'd be looking at narrowing it down to a specific area, although if coolant is getting on the cambelt I'd expect the leak to be more inboard than outboard. Although in saying that, a poor fitting top hose would spray inboard due to the way it fits.

Transfer pipe to top hose, check it's fully home/sealing ok?
Transfer pipe to stat, is it a good fit?
Thermostat itself, is there a chance that the seal moved out the groove on fitment, is it the correct seal, if new bolts were used are they the same length or have they bottomed out, were the faces of fitment clean, are the stat bodies the same thickness requiring different bolts etc etc.
It's got to come apart by the sound of it so all easily checkable and will hopefully become more apparent when it is apart. As for the B bolt, try it on its own first as already suggested. You'll know for sure then.

I'm sure you'll sort it.
