Not disagreeing with the thrust of your post, Roland, but just because they're not signatories doesn't mean that they can't be considered in breach of international law; the argument (and the lawyers will argue...) is that it is customary international law, since the majority of states have agreed, either in declaratory or tacit form that CW are beyond the pale and thus illegal.
The difficulty, of course, is that international law is not quite the fixed, immovable corpus that some assume it to be; it evolves through precedent, custom, treaty, agreement, etc, etc. This is why you have the debate about Responsibility To Protect: is it part of international law, or is it a legitimate approach which may - on occasion - appear to conflict with international law (arguably see Kosovo).
If Assad did use CW, he can't say that the use was aimed against the rebels and that the other 300, 400, however many that figure is now were collateral damage; indeed, that argument falls over under Additional Protocol 1 (1977) to the 1949 Convention. It doesn't fit in with Articles 51 and 57, since CW lack the discrimination required under those articles:
Article 51...
...4. Indiscriminate attacks are prohibited. Indiscriminate attacks are:
(a) those which are not directed at a specific military objective;
(b) those which employ a method or means of combat which cannot be directed at a specific military objective; or
(c) those which employ a method or means of combat the effects of which cannot be limited as required by this Protocol;
and consequently, in each such case, are of a nature to strike military objectives and civilians or civilian objects without distinction...
5. Among others, the following types of attacks are to be considered as indiscriminate:
(a) an attack by bombardment by any methods or means which treats as a single military objective a number of clearly separated and distinct military objectives located in a city, town, village or other area containing a similar concentration of civilians or civilian objects;
and
(b) an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.
Article 57...
2. With respect to attacks, the following precautions shall be taken:
(a) those who plan or decide upon an attack shall:
(i) do everything feasible to verify that the objectives to be attacked are neither civilians nor civilian objects and are not subject to special protection but are military objectives within the meaning of paragraph 2 of Article 52 and that it is not prohibited by the provisions of this Protocol to attack them;
(ii) take all feasible precautions in the choice of means and methods of attack with a view to avoiding, and in any event to minimizing, incidental loss or civilian life, injury to civilians and damage to civilian objects;
(iii) refrain from deciding to launch any attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated;
I suggest that CW fail to meet 57:2:ii and are insufficiently discriminating as the context of their use in Syria meant that it was almost certain that civilians were going to be gassed as part of the process, and the users of the CW failed to meet their responsibilities.
That doesn't necessarily mean that this demands the appearance of a large number of TLAMs in Syria airspace, though, simply that there are at least reasonable grounds to contend that the use of CW is a clear breach of the 1949 Convention and Additional Protocol 1.
The point, as you note, though, is that a case can be made to say that a number of elements of the civil war that did not see the use of CW contravene the Convention and AP1.
[Devil's Advocate]
To which some might claim that intervention should've occurred some time ago and that measured against the Convention, AP1 and the notions of R2P, the UN and international community have fallen down on their obligations and that Putin and to a lesser extent the Chinese have been particularly egregious in not living up to what they are expected to do...
[\Devil's Advocate]