Though this would be counterbalanced to a degree by the need for a reasonable level of chemical industry to produce large quantities of most agents.
Simplicity is a bit relative. You're right, not easy, but by comparison to nukes, dead easy. It suggests two things: more nations with CW arsenals &, coincidentally, a safer world than with nukes. If more nations have got 'em, & are willing to use 'em, it would seem to discourage anybody from doing anything rash.
OTGH,

the prospect of terrorists (of whatever stripe) having easy access to CW...

I was thinking of having the Wickham expedition to Brazil (an attempt to break the Brazilian control of the rubber market) in 1876 fail and alert the Brazilians to the dangers of bio-piracy. Hence there are no British rubber plantations created and supply is restricted to a degree. Of course eventually Brazil would lose control of the trade but it could last another few decades (I'm thinking of Fawcett's expedition in 1906 being another try) with the economic benefits (it was a very profitable trade) and social consequences (slavery and brutality, land clearance). Maybe they move to a plantation system for the Para trees and increase production dramatically (rather than tapping naturally isolated trees). Mix in Fordlandia a few decades later.
Perhaps it'd make a better RPG scenario than timeline.
Yeah, I see an RPG module or story (film?) less than a full TL.
I also see it being more probable than less: given the value of rubber, I'd picture more countries trying to steal, along with trying to develop artificial rubber. Work in that area has all manner of interesting knock-ons, from better tires to neoprene wet suits to...IDK what all.
There certainly seemed to be a greater adherence to such agreements back then. It's also possible that it's seen as "uncivilised", fine for use against "natives" but not allowed against more "civilised" enemies. In much the same way as expanding ammunition was OK for use in colonies but never really used in (say) WW1.
That's true. I wonder how much is *MAD: when your enemy has the ability to answer in kind, & perhaps more than, it tends to deter...
I really doubt agreements would be that comprehensive. Once a country has industrialised it's quite capable of manufacturing cyanides, arsenites, mustard, phosgene et cetera itself,
True. I guess I'm thinking more of limits on access by the likes of AQ (or, TTL, the IRA or FLQ), or Libya or North Korea.