I tend to disagree, but anybody please correct me if I’m wrong. 
The commentary doesn’t give the case but the meaning of the word. Chandaso clearly has an instrumental-like meaning, because it can be interpreted as “with consent/wish [to have intercourse]”. However, that does not make the word itself an instrumental.
According to Wijesekera the -so ending “is itself no case-ending at all”. So then it can not be an instrumental. Nor is it an ablative, technically. It “is however regarded as an ablatival form by Pāli grammarians” (meaning the ancient grammarians). I can’t remember any word with this ending functioning as a true ablative noun, though.
So on second thought I think -so is better regarded as a pure adverbial suffix, not derived from any particular case, similar to -tra for example. That would agree with Duroiselle paragraph 531(g): “Suffixes so and sā likewise form adverbs”, and he separates these adverbs from the case-form ones. (Unlike Wijesekera who despite saying it is no case ending discusses it under the ablative.)
Even adverbs which clearly stem from a case (e.g. santike) I prefer to class as indeclinables, which, not declining, technically don’t have a case.
With -so, Nyanatusita’s analysis of the patimokkha glosses in a similar vein: “antamaso: even so much as, even; indecl[inable]. Originally an ablative of anta[ma].” I.e. it originally may have been an ablative (although Wijesekera and Duroiselle may doubt that), but it now functions as an indeclinable, so has no case. Likewise, the Digital Pali Dictionary gives words like yoniso, bahuso, and dīghaso as indeclinables. It doesn’t include chandaso, but I think it can be classed similarly.
I had trouble explaining this in my Pali classes too, partly because grammarians tend to talk differently about these case-derived adverbs. Some say they have a case; others, like Duroiselle, say adverbs per definition don’t have a case. Also it is sometimes hard to say whether a word is an adverb or a noun with a case. But with chandaso I think it clearly is a pure adverb, just like “willingly” in English, even though it’s derived from a noun ‘will’.
Part of the problem of trilinear translations is that sometimes you have to/want to classify things strictly which perhaps aren’t strictly classifiable. Wish Pali was like mathematics, where definitions are clear cut and indisputable. 